[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