swift234

IRGen: Distinguish between 'class has fixed field count' and 'class has fixed size'

Subclasses of imported Objective-C classes have an unknown size, but
the start of the class's fields in the field offset vector is fixed,
since the field offset vector only contains offsets of Swift stored
properties. So we can always access fields with NonConstantDirect
(for concrete) or ConstantIndirect (for generic types).

On the other hand, generic subclasses of resilient classes must use
the most general NonConstantIndirect access pattern, because we can
add new fields resiliently.

Also, assume NSObject won't change size or grow any new instance
variables, allowing us to use the ConstantDirect access pattern.