[VFS] Add print/dump to the whole FileSystem hierarchy

For now most are implemented by printing out the name of the filesystem,
but this can be expanded in the future. Only `OverlayFileSystem` and
`RedirectingFileSystem` are properly implemented in this patch.
  - `OverlayFileSystem`: Prints each filesystem in the order that any
    operations are actually run on them. Optionally prints recursively.
  - `RedirectingFileSystem`: Prints out all mappings, as well as the
    `ExternalFS`. Most of this was already implemented other than the
    handling for the `DirectoryRemap` case and to actually print out the
    mapping.

Each FS should implement `printImpl` rather than `print`, where the
latter just fowards to the former. This is to avoid spreading the
default arguments through to the subclasses (where we may miss updating
in the future).

Differential Revision: https://reviews.llvm.org/D121421