[VFS] Add working directories to every virtual file system.

For RealFileSystem this is getcwd()/chdir(), the synthetic file systems can
make up one for themselves. OverlayFileSystem now synchronizes the working
directories when a new FS is added to the overlay or the overlay working
directory is set. This allows purely artificial file systems that have zero
ties to the underlying disks.

Differential Revision: http://reviews.llvm.org/D13430

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249316 91177308-0d34-0410-b5e6-96231b3b80d8