swift234

IRGen: Support polymorphic @objc_method functions

The type of the self parameter of an @objc_method might be a
BoundGenericClassType or a GenericTypeParamType. In these cases
we have to bind metadata.

For an @objc protocol P, the calling convention for an existential P
would try to pass type metadata in an extra argument, just like it
would for a <T : P> T. However, it is only necessary in the latter
case, because only there we can reflect on T to observe the statically
bound generic type. Fix this with a small hack so that ObjC protocol
methods don't get an extra argument for Self.

This patch alone doesn't enable any new functionality by itself,
because we hit the 'unimplemented dynamic layout' error instead of
crashing.

Swift SVN r29258