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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't think this is a good idea, warning on EVERY use of this is incorrect in system headers.  IT is going to result in a ton of we-want-to-be-suppressed-positives

I also don't think we can have a heuristic that wouldn't have an absurd amount of false positives.

IMO, the only thing we COULD do here is to have some sort of `#pragma clang` that wraps the template and says "show this even in system headers", which is an opt-in at the library level.

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