[clang-tidy] Add -vfsoverlay flag
It allows to remap and override files and directories on disk when
running clang-tidy. The intended use case for the flag is running
standalone clang-tidy binary for IDE and editor integration.
Patch by Vladimir Plyashkun.
Reviewers: alexfh, benlangmuir, ilya-biryukov
Reviewed By: ilya-biryukov
Subscribers: ilya-biryukov, cfe-commits
Differential Revision: https://reviews.llvm.org/D41535