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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Ah, that makes sense. It's surprising it wasn't like that in the first place

But can we now get rid of the -one-as sync scopes and replace them with MMRAs? 

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