Change return type of named swizzles

This is change 5 of 9 that fix problems with the specification of the
`vec` class.  An implementation that follows the existing specification
would not accept common code patterns and would not pass the CTS.  None
of the existing implementations actually follow the existing
specification.

This change aligns the return type of the 1-element "named" swizzles to
the behavior of the existing implementations (and the CTS).  This
changes the specified behavior of code like this:

```
vec<uint8_t, 4> v4{255};
auto v = v4.x();         // Returns reference to "uint8_t" not 1-element swizzle
int i = v4.x() + 1;      // Result is 256 not 0
```

These changes correspond to slides 19 - 22 of the presentation that was
discussed in the WG meetings.

adoc/chapters/programming_interface.adoc

@@ -17937,14 +17937,12 @@ The [code]#swizzleIndexes# argument pack specifies the elements in the swizzle.
 a@
 [source]
 ----
-__writeable_swizzle__ XYZW_ACCESS()
-__const_swizzle__ XYZW_ACCESS() const
+DataT& XYZW_ACCESS()
+const DataT& XYZW_ACCESS() const
 ----
    a@ Available only when: [code]#+NumElements <= 4+#.
 
-Return an instance of the implementation-defined [code]#+__writeable_swizzle__+#
-or [code]#+__const_swizzle__+# class representing a swizzled view of the vector
-as described in <<swizzled-vec-class>>.
+Return a reference to the element identified by [code]#XYZW_ACCESS#.
 
 Where [code]#XYZW_ACCESS# is: [code]#x# for [code]#NumElements == 1#,
 [code]#x, y# for
@@ -17957,27 +17955,22 @@ Where [code]#XYZW_ACCESS# is: [code]#x# for [code]#NumElements == 1#,
 a@
 [source]
 ----
-__writeable_swizzle__ RGBA_ACCESS()
-__const_swizzle__ RGBA_ACCESS() const
+DataT& RGBA_ACCESS()
+const DataT& RGBA_ACCESS() const
 ----
    a@ Available only when: [code]#NumElements == 4#.
 
-Return an instance of the implementation-defined [code]#+__writeable_swizzle__+#
-or [code]#+__const_swizzle__+# class representing a swizzled view of the vector
-as described in <<swizzled-vec-class>>.
+Return a reference to the element identified by [code]#RGBA_ACCESS#.
 
 Where [code]#RGBA_ACCESS# is: [code]#r, g, b, a#.
 
 a@
 [source]
 ----
-__writeable_swizzle__ INDEX_ACCESS()
-__const_swizzle__ INDEX_ACCESS() const
+DataT& INDEX_ACCESS()
+const DataT& INDEX_ACCESS() const
 ----
-  a@ Return an instance of the implementation-defined
-     [code]#+__writeable_swizzle__+# or [code]#+__const_swizzle__+# class
-     representing a swizzled view of the vector as described in
-     <<swizzled-vec-class>>.
+a@ Return a reference to the element identified by [code]#INDEX_ACCESS#.
 
 Where [code]#INDEX_ACCESS# is: [code]#s0# for [code]#NumElements == 1#,
 [code]#s0, s1# for
@@ -18712,43 +18705,62 @@ swizzle operation.
 a@
 [source]
 ----
-template <int... swizzleIndexes> __writeable_swizzle__</*unspecified*/> swizzle() const  (1)
+DataT& XYZW_ACCESS() const         (1)
+DataT& RGBA_ACCESS() const         (2)
+DataT& INDEX_ACCESS() const        (3)
 
-__writeable_swizzle__</*unspecified*/> XYZW_ACCESS() const                               (2)
-__writeable_swizzle__</*unspecified*/> RGBA_ACCESS() const                               (3)
-__writeable_swizzle__</*unspecified*/> INDEX_ACCESS() const                              (4)
+const DataT& XYZW_ACCESS() const   (4)
+const DataT& RGBA_ACCESS() const   (5)
+const DataT& INDEX_ACCESS() const  (6)
+----
+|====
+_Availability:_ Functions (1) - (3) are available only in
+[code]#+__writeable_swizzle__+#.
+Functions (4) - (6) are available only in [code]#+__const_swizzle__+#.
+
+_Constraints:_ These functions have the same constraints as the equivalent
+member functions of the [code]#vec# class.
+
+_Returns:_ A reference to the element of the underlying [code]#vec# object that
+corresponds to the position of the swizzle operation identified by
+[code]#XYZW_ACCESS#, [code]#RGBA_ACCESS#, or [code]#INDEX_ACCESS#.
+
+'''
+
+[frame=all,grid=none,separator="@"]
+|====
+a@
+[source]
+----
+template <int... swizzleIndexes> __writeable_swizzle__</*unspecified*/> swizzle() const  (1)
 
 #ifdef SYCL_SIMPLE_SWIZZLES
-__writeable_swizzle__</*unspecified*/> XYZW_SWIZZLE() const                              (5)
-__writeable_swizzle__</*unspecified*/> RGBA_SWIZZLE() const                              (6)
+__writeable_swizzle__</*unspecified*/> XYZW_SWIZZLE() const                              (2)
+__writeable_swizzle__</*unspecified*/> RGBA_SWIZZLE() const                              (3)
 #endif
 
-__writeable_swizzle__</*unspecified*/> lo() const                                        (7)
-__writeable_swizzle__</*unspecified*/> hi() const                                        (8)
-__writeable_swizzle__</*unspecified*/> odd() const                                       (9)
-__writeable_swizzle__</*unspecified*/> even() const                                      (10)
-
+__writeable_swizzle__</*unspecified*/> lo() const                                        (4)
+__writeable_swizzle__</*unspecified*/> hi() const                                        (5)
+__writeable_swizzle__</*unspecified*/> odd() const                                       (6)
+__writeable_swizzle__</*unspecified*/> even() const                                      (7)
 
-template <int... swizzleIndexes> __const_swizzle__</*unspecified*/> swizzle() const      (11)
 
-__const_swizzle__</*unspecified*/> XYZW_ACCESS() const                                   (12)
-__const_swizzle__</*unspecified*/> RGBA_ACCESS() const                                   (13)
-__const_swizzle__</*unspecified*/> INDEX_ACCESS() const                                  (14)
+template <int... swizzleIndexes> __const_swizzle__</*unspecified*/> swizzle() const      (8)
 
 #ifdef SYCL_SIMPLE_SWIZZLES
-__const_swizzle__</*unspecified*/> XYZW_SWIZZLE() const                                  (15)
-__const_swizzle__</*unspecified*/> RGBA_SWIZZLE() const                                  (16)
+__const_swizzle__</*unspecified*/> XYZW_SWIZZLE() const                                  (9)
+__const_swizzle__</*unspecified*/> RGBA_SWIZZLE() const                                  (10)
 #endif
 
-__const_swizzle__</*unspecified*/> lo() const                                            (17)
-__const_swizzle__</*unspecified*/> hi() const                                            (18)
-__const_swizzle__</*unspecified*/> odd() const                                           (19)
-__const_swizzle__</*unspecified*/> even() const                                          (20)
+__const_swizzle__</*unspecified*/> lo() const                                            (11)
+__const_swizzle__</*unspecified*/> hi() const                                            (12)
+__const_swizzle__</*unspecified*/> odd() const                                           (13)
+__const_swizzle__</*unspecified*/> even() const                                          (14)
 ----
 |====
-_Availability:_ Functions (1) - (10) are available only in
+_Availability:_ Functions (1) - (7) are available only in
 [code]#+__writeable_swizzle__+#.
-Functions (11) - (20) are available only in [code]#+__const_swizzle__+#.
+Functions (8) - (14) are available only in [code]#+__const_swizzle__+#.
 
 _Constraints:_ These functions have the same constraints as the equivalent
 member functions of the [code]#vec# class.

adoc/headers/vec.h

@@ -83,18 +83,18 @@ template <typename DataT, int NumElements> class vec {
 
   // Available only when NumElements <= 4.
   // XYZW_ACCESS is: x, y, z, w, subject to NumElements.
-  __writeable_swizzle__ XYZW_ACCESS();
-  __const_swizzle__ XYZW_ACCESS() const;
+  DataT& XYZW_ACCESS();
+  const DataT& XYZW_ACCESS() const;
 
   // Available only NumElements == 4.
   // RGBA_ACCESS is: r, g, b, a.
-  __writeable_swizzle__ RGBA_ACCESS();
-  __const_swizzle__ RGBA_ACCESS() const;
+  DataT& RGBA_ACCESS();
+  const DataT& RGBA_ACCESS() const;
 
   // INDEX_ACCESS is: s0, s1, s2, s3, s4, s5, s6, s7, s8, s9, sA, sB, sC, sD,
   // sE, sF, subject to NumElements.
-  __writeable_swizzle__ INDEX_ACCESS();
-  __const_swizzle__ INDEX_ACCESS() const;
+  DataT& INDEX_ACCESS();
+  const DataT& INDEX_ACCESS() const;
 
 #ifdef SYCL_SIMPLE_SWIZZLES
   // Available only when NumElements <= 4.