[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Aaron Ballman via cfe-commits]

================
@@ -2331,6 +2335,9 @@ def err_init_reference_member_uninitialized : Error<
   "reference member of type %0 uninitialized">;
 def note_uninit_reference_member : Note<
   "uninitialized reference member is here">;
+def warn_field_requires_explicit_init : Warning<
+  "field %select{%1|in %1}0 is not explicitly initialized, but was marked as "
+  "requiring explicit initialization">, InGroup<UninitializedExplicitInit>;
----------------
AaronBallman wrote:

```suggestion
  "field %select{%1|in %1}0 requires explicit initialization but is not explicitly initialized">,
   InGroup<UninitializedExplicitInit>;
```

https://github.com/llvm/llvm-project/pull/102040