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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Thank you for this fix, I think it makes sense but this really requires a release note and it should be detailed so users can understand the impact.
> 
> I would have preferred you waited for one of the frontend folks to review before landing. You can always ping us on discord.

Sorry. I will wait longer. Did you mean pings on `#clang`?

I decided to create a release/22.x backport with release note change https://github.com/llvm/llvm-project/pull/188422

The rationale is that otherwise ` int main() { return __COUNTER__; }`, which compiles fine for `clang-21 -pedantic-errors`, will error clang 22 with no way to downgrade the diagnostic to a warning.

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