swift234

Split the "Foreign" flag into a ForeignKind enum.

This flag tracks whether we have a special kind of imported class
that has limitations in what you can do with it. Currently it's
used for two things: CF classes, and the magic "Protocol" class used
to represent Objective-C protocol metadata. I'm planning to add a
third to handle classes with the recently-added objc_runtime_visible
attribute, which describes an Objective-C class whose runtime symbols
are hidden (forcibly preventing categories and subclassing). This is
used for some of the types in Dispatch, which has exposed some of the
classes that were considered implementation details on past OSs.

I'm splitting the flag into an enum rather than just marking the
Dispatch classes with the existing flag because we still need to
be able to /cast/ to the Dispatch types (which you can't do with CF
types today) and because they deserve better than to be lumped in
with CF for diagnostic purposes.

Groundwork for rdar://problem/26850367, which is that Swift will
happily let you extend the new Dispatch classes but then fails to find
the symbols at link-time.