[clang] [AMDGPU] Add builtin to test for constant memory (PR #209679)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm requested changes to this pull request.

Constant address space is entirely synthetic and shouldn't be directly observable. What is the point of this API, and why do we need compatibility now?

https://github.com/llvm/llvm-project/pull/209679