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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Testcase?

This seems like the sort of thing where we might want to cooperate with the C++ library somehow to get better diagnostics, but getting some diagnostic seems like an improvement.

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