[clang-tidy] Fix clang-tidy doesn't read .clangtidy configuration file.

Summary: Fix https://bugs.llvm.org/show_bug.cgi?id=34900.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: JonasToth, klimek, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D45697

llvm-svn: 330245