[clang] [CIR][AMDGPU] Implement the clang::atomic statement attribute (PR #225364)

[Steffen Larsen via cfe-commits]

steffenlarsen wrote:

@bcardosolopes @andykaylor @xlauko - The changes here are mostly AMDGPU, but since it is bigger I would prefer to also have an approval from one of you, even if just a rubber stamp.

https://github.com/llvm/llvm-project/pull/225364