[clang][HeaderSearch] Shorten paths for includes in mainfile's directory

Summary:
Currently HeaderSearch only looks at SearchDir's passed into it, but in
addition to those paths headers can be relative to including file's directory.

This patch makes sure that is taken into account.

Reviewers: gribozavr

Subscribers: jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63295

llvm-svn: 365005