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

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> Perhaps we need a stripped down OpenCL header, like `clc/opencl/opencl-base.h` which includes just the types and macro defs?

See #146840 for an example of this approach.

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