[clang] [AMDGPU] Diagnose unaligned atomic (PR #80322)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

> You may want to check that we can still disable the error with `-Wno-error=atomic-alignment` passed via top-level options.
> 
> Other than that LGTM.

Good point. Added a test

https://github.com/llvm/llvm-project/pull/80322