[clang-tidy] Add -vfsoverlay flag

Summary:
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

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D41535

llvm-svn: 323196