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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Looks fine, but I'd like a second approval; I haven't really looked at this template instantiation stuff recently.

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