If the ExternalFS has already remapped to an external path then
RedirectingFileSystem should not change it to the originally provided
path. This fixes the original path always being used if multiple VFS
overlays were provided and the path wasn't found in the highest (ie.
first in the chain).
For now this is accomplished through the use of a new
ExposesExternalVFSPath field on vfs::Status. This flag is true when
the Status has an external path that's different from its virtual
path, ie. the contained path is the external path. See the plan in
FileManager::getFileRef for where this is going - eventually we won't
need IsVFSMapped any more and all returned paths should be virtual.
Resolves rdar://90578880 and llvm-project#53306.