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

[via cfe-commits]

higher-performance wrote:

Are folks still on the fence after the latest discussion? If anyone still has objections I can try to suggest some path forward, but I'm not sure what everyone is thinking right now (or what this is pending on in general), and it would be lovely to move forward somehow.

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