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

[via cfe-commits]

================
@@ -29,6 +29,7 @@
 #include "clang/AST/TypeLoc.h"
 #include "clang/AST/UnresolvedSet.h"
 #include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/DiagnosticSema.h"
----------------
higher-performance wrote:

Are you saying I should move `warn_cxx20_compat_requires_explicit_init_non_aggregate` to that file?

To me it seems very similar to other diagnostics in `DiagnosticSemaKinds.td`, like `warn_cxx20_compat_aggregate_init_with_ctors`. I actually don't see other `warn_cxx*` warnings in there, are you sure it would belong better in there?

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