[clang] Make warning pragma override -Werror=foo and DefaultError warnings (PR #93647)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> I think we should update our documentation:
> 
> https://github.com/llvm/llvm-project/blob/b01ac5137c28fa5e1b44a5d850cb7a6ace7d8799/clang/docs/UsersManual.rst?plain=1#L1186
> 
> Overall, this seems like a sensible improvement to me and the code changes LG.

Thx. Updated UserManual.rst

https://github.com/llvm/llvm-project/pull/93647