Port some stage1 test cases to stage2

There are now very few stage1 cases remaining:
* `cases/compile_errors/stage1/obj/*` currently don't work correctly on
  stage2. There are 6 of these, and most of them are probably fairly
  simple to fix.
* `cases/compile_errors/async/*` and all remaining `safety/*` depend on
  async; see ziglang#6025.

Resolves: ziglang#14849

compile_errors/undefined_as_field_type_is_rejected.zig

@@ -7,7 +7,7 @@ export fn entry1() void {
 }
 
 // error
-// backend=stage1
+// backend=stage2
 // target=native
 //
-// tmp.zig:2:8: error: use of undefined value here causes undefined behavior
+// :2:8: error: use of undefined value here causes undefined behavior