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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

I want to be able to split the build to produce multiple Bitcode libraries instead of the monolithic libclc.bc. e.g., we need to split out the images since not all subtargets support them. Plus we need 2 different builds of the image library 

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