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

[Wenju He via cfe-commits]

wenju-he 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.

thanks @frasercrmck I'll update OpenCL headers of this PR after #146840 is landed.

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