[libclc] [libclc] Use custom CMake handling to overhaul libclc compilation (PR #185247)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I added some more, does this fix the ones you're missing? I had to rebase this a few times so some things might've gotten misplaced.

https://github.com/llvm/llvm-project/pull/185247