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

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b-sumner wrote:

OpenCL should only be using the amdgcn fence builtin.  See, e.g. https://github.com/ROCm/llvm-project/blob/amd-staging/amd/device-libs/opencl/src/misc/awif.cl#L73 

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