[libclc] [libclc][NFC] De-duplicate subgroup workitem function decls (PR #189328)

[Wenju He via cfe-commits]

wenju-he wrote:

> would be better served by building all of the components into libraries and letting the linker take handle it

Only kept header changes in this PR. Implementation merging of generic/nvptx files can be done when above approach is in place.

https://github.com/llvm/llvm-project/pull/189328