[clang] [Clang][AMDGPU] Stop defaulting to `one-as` for all atomic scopes (PR #120095)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

I would say this change is obviously correct, but I can't see why it was introduced and vaguely fear tripping over abhorrent behaviour in the backend. Can you send this down the internal CI pipeline to pick up some more runtime testing (unless amd-stg-open is already defaulting to the right thing?)

https://github.com/llvm/llvm-project/pull/120095