swift234

IRGen: Fix layout of concrete subclasses of generic subclasses of imported Objective-C classes

class B<T> : NSFoo {}
class A : B<Int> {}

IRGen computes the ivar layout starting from offset zero, since
the size of the 'NSFoo' is unknown and we rely on the Objective-C
runtime to slide the ivar offsets.

The instantiated metadata for B<Int> would contain a field offset
vector with the correct offsets, because of how
swift_initClassMetadata_UniversalStrategy() works.

However, A's metadata is emitted statically, and this includes a
copy of the field offset vector from the superclass. A's metadata
was initialized by swift_initializeSuperclass(), which did not
copy the field offset vector over from A<Int>. And since the
Objective-C runtime only slides the immediate ivars of a class,
the field offsets corresponding to A<Int>'s fields in B's type
metadata were never slid, resulting in problems when an instance
of B was passed to a function operating on an A<T> generically.

Fixes <rdar://problem/23200051>.