[libclc] [libclc] Declare workitem built-ins in clc, move ptx-nvidiacl workitem built-ins into clc (PR #144333)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Moving them into clc should resolve the concern, but I don't know where is destination directory in clc. I don't know how to map `opencl/lib/amdgcn` and `opencl/lib/r600` to `clc/lib/amdgcn` and `clc/lib/amdgpu`. Do you have suggestions about the mapping?

r600 and amdgcn should be treated as separate targets. we should stop trying to have common code under an "amdgpu" name

https://github.com/llvm/llvm-project/pull/144333