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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> 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

This should be doable and this should make creating new libraries much easier since we don't need to use a bunch of specially made functions anymore.

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