[clang] Include [[clang::require_explicit_initialization]] warnings in system headers (PR #141133)

[via cfe-commits]

higher-performance wrote:

Got it, thanks for the update! Okay, let's see what they think when they have a chance to reply. If there's still disagreement, I'll try to suggest some other ways to move forward.

https://github.com/llvm/llvm-project/pull/141133