Sema: fix resolution of inferred error sets

Introduce `Module.ensureFuncBodyAnalyzed` and corresponding `Sema`
function. This mirrors `ensureDeclAnalyzed` except also waits until the
function body has been semantically analyzed, meaning that inferred
error sets will have been populated.

Resolving error sets can now emit a "unable to resolve inferred error
set" error instead of producing an incorrect error set type. Resolving
error sets now calls `ensureFuncBodyAnalyzed`. Closes #11046.

`coerceInMemoryAllowedErrorSets` now does a lot more work to avoid
resolving an inferred error set if possible. Same with
`wrapErrorUnionSet`.

Inferred error set types no longer check the `func` field to determine if
they are equal. That was incorrect because an inline or comptime function
call produces a unique error set which has the same `*Module.Fn` value for
this field. Instead we use the `*Module.Fn.InferredErrorSet` pointers to
test equality of inferred error sets.

test/behavior/array.zig

@@ -538,9 +538,6 @@ test "type coercion of anon struct literal to array" {
             try expect(arr1[1] == 56);
             try expect(arr1[2] == 54);
 
-            if (builtin.zig_backend == .stage2_llvm) return error.SkipZigTest; // TODO
-            if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-
             var x2: U = .{ .a = 42 };
             const t2 = .{ x2, .{ .b = true }, .{ .c = "hello" } };
             var arr2: [3]U = t2;