[llvm] [RFC][AMDGPU] Add vulkan:private/nonprivate MMRAs support (PR #78573)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Not ready for review, I just rebased it but I need to go back to this and verify all the semantics are correct, and write the memory model documentation as well.

https://github.com/llvm/llvm-project/pull/78573