swift234

IRGen: Simplify generated code for generic class metadata instantiation

Move the following from IRGen to runtime:

- Copying generic parameters from superclass to subclass
- Copying field offsets from superclass to subclass
- Initializing the Objective-C runtime name of the subclass

This eliminates some duplication between the generic subclass and
concrete subclass of a generic class cases.

Also this should reduce generated code size and have no impact on
performance (the instantiation logic only runs once per substituted
type).