Revert "[clang-tidy] Fix relative path in header-filter."

This reverts commit r372388. It made '-header-filter' inconsistent with
paths printed in diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@372601 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed