[clang] [clang][Driver] Fix libc++ include path on FreeBSD (PR #212725)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @aokblast Can you give the patch a whirl now the testsuite issues are resolved? Thanks.
> 
> Actually, getting the Unix vs. Windows directory separator handling right was extremely tedious and time consuming. There must be a better way than littering every affected test with the likes of `[[SEP:/|\\\\]]`: this makes the test cases close to unreadable. I'll file an issue about this.

We use `UNSUPPORTED: systme-windows` in some clang/test/Driver tests where the windows portability becomes tedious...

https://github.com/llvm/llvm-project/pull/212725