[clang] [clang] Adjust -pedantic-errors -WX/-Wno-error=X interaction (PR #184756)

[Harald van Dijk via cfe-commits]

hvdijk wrote:

> with no way to downgrade the diagnostic to a warning

The rationale doesn't quite hold up since as you say, `-pedantic-errors -Wlong-long` did work for that, but it's not reasonable to have users use that when it works the other way around in GCC, it's much better to have that backport IMO.

https://github.com/llvm/llvm-project/pull/184756