[clang] [llvm] [SPIR-V] Encode Atomic metadata as UserSemantic string decoration (PR #193019)

[Dmitry Sidorov via llvm-commits]

https://github.com/MrSidims approved this pull request.

I'll take AR to get out some NonSemantic instruction set that we could reuse later.

https://github.com/llvm/llvm-project/pull/193019