[clang] [llvm] [AMDGPU] Add amdgpu-as MMRA for fences (PR #78572)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I thought image memory = private. It's unclear to me, what AS does OpenCL IMAGE memory map to in our backend? (But otherwise, yes, MMRA should just have the backend names, the mapping of the OpenCL IMAGE to a backend AS should be in the device-lib)

Images are global memory with magical addressing and value interpretation on load/store. There's nothing private about them 

https://github.com/llvm/llvm-project/pull/78572