Initial implementation of virtual file system

This adds the minimum virtual file system support to start migrating
FileManager onto the VFS.

Originally discussed here:
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-February/035188.html

Differential Revision: http://llvm-reviews.chandlerc.com/D2745

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201618 91177308-0d34-0410-b5e6-96231b3b80d8