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

[Rainer Orth via cfe-commits]

rorth wrote:

> > Wouldn't removing `FreeBSD::addLibCxxIncludePaths()` just fallback to `Generic_GCC::addLibCxxIncludePaths()`?
> 
> No idea: my C++ is very weak TBH.

Seems to work indeed: I'll do just another round of testing, then adjust the tests.

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