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

[Karol Herbst via cfe-commits]

karolherbst wrote:

The base header enables a bunch of extensions when `__SPIRV__` or `__SPIR__` is found, and that define is set automatically when a SPIRV or SPIR tripple is used.

Maybe there is a better tripple to be used in this case? However we do want to compile to SPIR-V (with the native target in the future anyway), it's just it enables a bunch of things automatically.

I think it's fine to do it that way, it's just a bit of surprising behavior if you want to have tight control over what extensions are enabled or not.

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