[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:

Is an issue here perhaps that downstream users may have implemented (e.g.) `get_global_offset` in their toolchains, but after this change they'll suddenly see a definition of this function that they're not expecting?

I suppose they would have to implement CLC functions, like `__clc_get_global_offset` to work with this approach. Or they could have their libclc target not call into `__clc_get_global_offset`, or just not provide a definition of `get_global_offset` as before. Either way this has the potential to be a breaking change.

I'm not sure about leaving dangling CLC declarations in OpenCL libraries. It feels like an implementation detail we're exposing to users.

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