[clang] [Clang][Modules] Ensure global diagnostic overrides are respected in system modules (PR #180684)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem approved this pull request.

> Instead of checking the SuppressSystemWarnings bit of the current DiagState, I introduced a new `ForceSystemWarnings` flag in DiagnosticsEngine that lives outside of the DiagState mechanism.

Thanks! I think this is a good fix.

LGTM modulo some comment nits.

https://github.com/llvm/llvm-project/pull/180684