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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

This one-as business seems like it's cruft from before MMRAs. Can we rip them out and replace them with MMRAs for OpenCL? 

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