[libclc] [libclc][NFC] Prioritize clang's built-in opencl-c-base.h over CPATH versions (PR #177754)

[Matt Arsenault via cfe-commits]

arsenm wrote:

clang's header should obviously win, but I'm puzzled why this needs to be handled here in cmake. I'm surprised anything has priority over the resource directory 

https://github.com/llvm/llvm-project/pull/177754