[clang][lld][cmake] Simplify header dirs

We don't need to recompute the list LLVMConfig.cmake provides us.

When LLVM is being built, the list is two elements long: generated headers and headers from source.

When LLVM is already built,the list is one element long: the installed header directory containing both generated and hand-written sources.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D130553