[clang] [Clang] Include clang standard lib header directory from Linux (PR #175593)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I reworked this to apply this behavior to all Linux, as I think it's reasonable that we search the clang standard include directory as a part of our toolchain. This serves the original purpose but should make this more generic. PTAL.

https://github.com/llvm/llvm-project/pull/175593