[clang] [Clang] Include LLVM libc header directory from Linux (PR #175593)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Test failure is relevant to the changes, I think it might be due to regex using the wrong path separators on Windows, but I didn't poke too hard into what was going on.

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