We add a new runtime entry point, swift_dynamicCastUnconditional(),
that aborts when the cast fails. We'll probably want to throw an
exception in the future, but this is fine for now.
Swift SVN r3554
Notably, there is still no support for +1 return values,
so we'll leak when doing alloc/init and so on; but this gets
the fundamentals in place. A lot of the extra stuff in here
is dealing with mapping between metatypes and class objects.
Swift SVN r3425
The interesting thing here is that we need runtime support in
order to generate references to metatypes for classes, mostly
because normal ObjC classes don't have all the information we want
in a metatype (which for now just means the VWT pointer).
We'll need to be able to reverse this mapping when finding a
class pointer to hand off to, say, an Objective-C class method,
of course.
Swift SVN r3424
