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

[Alexey Bader via cfe-commits]

bader wrote:

> I'm still curious who or what is setting `CPATH` here

I'm using the same environment to build LLVM and a couple of other projects. Probably third-party projects injected `CPATH` into my environment. I'll explore the option with clearing `CPATH` value for building LLVM. Thanks!

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