Pattern completeness: Handle structures under union constructors

[Alasdair]

The following now generates a correct counterexample:

enum E = {A, B}

struct Foo = { bar : E }

val main : unit -> unit

function main() = {
    let x : Foo = struct { bar = B };
    match Some(x) {
      Some(struct { bar = A }) => (),
      None()  => (),
    }
}

whereas previously it would just mark the pattern as potentially incomplete and continue.

[github-actions]

Test Results

    9 files  ±0     21 suites  ±0   0s ⏱️ ±0s
  681 tests +1    681 ✅ +1  0 💤 ±0  0 ❌ ±0 
2 143 runs  +1  2 142 ✅ +1  1 💤 ±0  0 ❌ ±0 

Results for commit b905e9b. ± Comparison against base commit 8d4b9d8.

♻️ This comment has been updated with latest results.