[clang] [Clang][AMDGPU] Add ``amdgcn_av("none")`` attribute for atomic expressions (PR #199622)

[Sameer Sahasrabuddhe via cfe-commits]

ssahasra wrote:

This is now ready for review. Everything else is in place, and the Clang attribute introduced here is the final piece that makes it usable.

https://github.com/llvm/llvm-project/pull/199622