swift234

[IRGen] Functions with the ObjCMethod convention are not polymorphic.

...even if the 'self' type is generic. Additionally, Objective-C generic
types cannot be used as a source of type metadata, because Objective-C
generics are erased at runtime by default. (This may need to change.)

With these two changes, we now pass type metadata explicitly when we need
to, and /don't/ try to pass it to Objective-C methods that would have
needed it if they were Swift methods.