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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

@b-sumner has useful context on this. I won't paraphrase, but it sounds like the block deleted here has the right semantics for opencl, where "seqcst" has some special meaning and generally the semantics don't totally make sense to me. Suggest we amend this to "if opencl, do this thing, otherwise leave it alone"

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