[VFS] Add support to RedirectingFileSystem for mapping a virtual directory to...

[VFS] Add support to RedirectingFileSystem for mapping a virtual directory to one in the external FS.

Previously file entries in the -ivfsoverlay yaml could map to a file in the
external file system, but directories had to list their contents in the form of
other file entries or directories. Allowing directory entries to map to a
directory in the external file system makes it possible to present an external
directory's contents in a different location and (in combination with the
'fallthrough' option) overlay one directory's contents on top of another.

rdar://problem/72485443
Differential Revision: https://reviews.llvm.org/D94844