[Driver] Fix libcxx detection on Darwin with clang run as ./clang

Summary:
By using '..' instead of fs::parent_path.

The intention of the code was to go from 'path/to/clang/bin' to
'path/to/clang/include'. In most cases parent_path works, however it
would fail when clang is run as './clang'.

This was noticed in Chromium's bug tracker, see
https://bugs.chromium.org/p/chromium/issues/detail?id=919761

Reviewers: arphaman, thakis, EricWF

Reviewed By: arphaman, thakis

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D56446

llvm-svn: 350714