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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Move subgroup workitem function implementations into individual files to align with other workitem functions.

Honestly I'd rather merge all of the other cases into one file than continue doing this. It's a lot of extra boilerplate and just burning build time to have one function per file. The only reason to maintain this is the source file based target overriding, which would be better served by building all of the components into libraries and letting the linker take handle it.

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