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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> > @arsenm Should we use `image` or `private`? We could allow both in the frontend, and only use `private` as the canonical MMRA.
> 
> I don't understand why image would imply private. I would just keep at as private throughout

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)

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