[libclc] [libclc] use default paths with find_program when possible (PR #105969)

[Tristan Ross via cfe-commits]

RossComputerGuy wrote:

> Yes, it should be built along with the core (but doesn't need to ship in the same package as the core).

Sounds good, I'll communicate with the other LLVM Nix maintainers and see if this is a good idea to fully go ahead on.

https://github.com/llvm/llvm-project/pull/105969