compiler: move unions into InternPool

There are a couple concepts here worth understanding:

Key.UnionType - This type is available *before* resolving the union's
fields. The enum tag type, number of fields, and field names, field
types, and field alignments are not available with this.

InternPool.UnionType - This one can be obtained from the above type with
`InternPool.loadUnionType` which asserts that the union's enum tag type
has been resolved. This one has all the information available.

Additionally:

* ZIR: Turn an unused bit into `any_aligned_fields` flag to help
  semantic analysis know whether a union has explicit alignment on any
  fields (usually not).
* Sema: delete `resolveTypeRequiresComptime` which had the same type
  signature and near-duplicate logic to `typeRequiresComptime`.
  - Make opaque types not report comptime-only (this was inconsistent
    between the two implementations of this function).
* Implement accepted proposal #12556 which is a breaking change.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/std/meta.zig b/lib/std/meta.zig
index 3bc5fc9864..b10ecd2731 100644
--- a/lib/std/meta.zig
+++ b/lib/std/meta.zig
@@ -614,9 +614,9 @@ test "std.meta.FieldEnum" {
     const Tagged = union(enum) { a: u8, b: void, c: f32 };
     try testing.expectEqual(Tag(Tagged), FieldEnum(Tagged));
 
-    const Tag2 = enum { b, c, a };
+    const Tag2 = enum { a, b, c };
     const Tagged2 = union(Tag2) { a: u8, b: void, c: f32 };
-    try testing.expect(Tag(Tagged2) != FieldEnum(Tagged2));
+    try testing.expect(Tag(Tagged2) == FieldEnum(Tagged2));
 
     const Tag3 = enum(u8) { a, b, c = 7 };
     const Tagged3 = union(Tag3) { a: u8, b: void, c: f32 };