swift234

Implement metatype references to ObjC classes.

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