[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:

I am seeing unresolved CLC functions in `nvptx--.bc` and `nvptx64--.bc`. I think it's because we're now building things like `get_num_groups` for `nvptx`/`nvptx64`, whereas previously they were not being built for those targets. We're also not building `__clc_get_num_groups` because we're (correctly) not including `ptx-nvidiacl` sources.

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