llvm: unions which are equivalent to enums are not by-ref

The LLVM backend lowers unions where all fields are zero-bit as
equivalent to their backing enum, and expects them to have the same
by-ref-ness in at least one place in the backend, probably more.

Resolves: #23577

src/codegen/llvm.zig

@@ -12777,7 +12777,7 @@ fn isByRef(ty: Type, zcu: *Zcu) bool {
         },
         .@"union" => switch (ty.containerLayout(zcu)) {
             .@"packed" => return false,
-            else => return ty.hasRuntimeBits(zcu),
+            else => return ty.hasRuntimeBits(zcu) and !ty.unionHasAllZeroBitFieldTypes(zcu),
         },
         .error_union => {
             const payload_ty = ty.errorUnionPayload(zcu);