[Tooling] Added a VFS parameter to ClangTool

Summary:
The parameter overrides the underlying vfs used by ClangTool for
filesystem operations.

Patch by Vladimir Plyashkun.

Reviewers: alexfh, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41947

llvm-svn: 323195