Also, move the ObjectiveC overlay into its own directory, so that we can
use the directory name as an indicator of what overlays exist.
This is in preparation for DevPubs providing the contents of the apinotes/
directory. The downside here is that adding new apinotes files doesn't
automatically trigger a rebuild; I intend to mitigate that somewhat by at
least triggering one when the revision number of the DevPubs repo changes.
Swift SVN r21078
