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

[Aaron Ballman via cfe-commits]

================
@@ -2302,6 +2302,10 @@ def err_init_list_bad_dest_type : Error<
 def warn_cxx20_compat_aggregate_init_with_ctors : Warning<
   "aggregate initialization of type %0 with user-declared constructors "
   "is incompatible with C++20">, DefaultIgnore, InGroup<CXX20Compat>;
+def warn_cxx20_compat_requires_explicit_init_non_aggregate : Warning<
+  "explicit initialization of field %0 may not be enforced in C++20 as type %1 "
+  "will become a non-aggregate due to the presence of user-declared "
+  "constructors">, DefaultIgnore, InGroup<CXX20Compat>;
----------------
AaronBallman wrote:

```suggestion
  "explicit initialization of field %0 will not be enforced in C++20 and later because %1 "
  "has a user-declared constructor, making the type no longer an aggregate">,
  DefaultIgnore, InGroup<CXX20Compat>;
```

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