Invalid code generation for global enum with Always_Valid => True

[jklmnn]

The code generation for enumeration types with the aspect Always_Valid => True generates invalid code if an object of such a type is used in the global context.

If the following patch is applied:

diff --git a/tests/integration/session_comprehension_on_sequence/test.rflx b/tests/integration/session_comprehension_on_sequence/test.rflx
index 7fe8739c..4cfc60d3 100644
--- a/tests/integration/session_comprehension_on_sequence/test.rflx
+++ b/tests/integration/session_comprehension_on_sequence/test.rflx
@@ -11,6 +11,7 @@ package Test is
       Options : Universal::Options; -- §S-D-V-T-MS, §S-D-V-E-N
       Message_1 : Universal::Message; -- §S-S-D-V-T-M, §S-S-D-V-E-N
       Message_2 : Universal::Message; -- §S-S-D-V-T-M, §S-S-D-V-E-N
+      First_Option_Type : Universal::Option_Type;
    begin
       state Start is
       begin
@@ -30,6 +31,7 @@ package Test is
          Option_Types : Universal::Option_Types; -- §S-S-D-V-T-SS, <C2><A7>S-S-D-V-E-N
          Message_Options : Universal::Options; -- §S-S-D-V-T-MS, <C2><A7>S-S-D-V-E-N
       begin
+         First_Option_Type := Universal::OT_Data;
          -- §S-S-A-A-LC, §S-E-LC-V, §S-E-LC-SMS, §S-E-LC-TSS, §S-E-LC-CS, §S-E-LC-A, §S-E-S-V
          Option_Types := [for E in Options if E.Option_Type = Universal::OT_Data => E.Option_Type];
          -- Test that target sequence is reset by assignment

This will make the following test fail:

python -m pytest 'tests/integration/feature_test.py::test_compilability[session_comprehension_on_sequence]' -vv

The reason is that the default initializer is generated the same way as for regular enums

First_Option_Type : Universal.Option_Type := Universal.Option_Type'First;

but the Ada type definition is a variant record. In this case no default initializer should be generated.