[libclc] [libclc] Stop installing CLC headers (PR #126908)

[Matt Arsenault via cfe-commits]

arsenm wrote:

I'm surprised any of the extension macros are defined in the builtin header, and they probably should not be there. The target info in clang records the extensions from the target, and I'd expect the predefined macros to be populated from there 

https://github.com/llvm/llvm-project/pull/126908