[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.

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