[clang-tidy] Make llvm-header-guard work on llvm git monorepo

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: xazax.hun, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68026

llvm-svn: 372953