Include [[clang::require_explicit_initialization]] warnings in system headers

Fixes #141103

Author: higher-performance

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2423,7 +2423,8 @@ def note_uninit_reference_member : Note<
   "uninitialized reference member is here">;
 def warn_field_requires_explicit_init : Warning<
   "field %select{%1|in %1}0 requires explicit initialization but is not "
-   "explicitly initialized">, InGroup<UninitializedExplicitInit>;
+   "explicitly initialized">, InGroup<UninitializedExplicitInit>,
+   ShowInSystemHeader;
 def warn_field_is_uninit : Warning<"field %0 is uninitialized when used here">,
   InGroup<Uninitialized>;
 def warn_base_class_is_uninit : Warning<