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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> CUDA documents `__isConstant` as an address-space predicate: https://docs.nvidia.com/cuda/archive/12.9.0/cuda-c-programming-guide/index.html#isconstant

This is kind of a terrible feature. Can we make the query a little broader, and just check what's in .rodata?




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