pass the size and alignment of each field. Take advantage
of this to pass a constant size and alignment when
possible.
This avoids the need to recursively find type metadata for
every field type, allowing generic recursively-structured
classes to be built. There are a number of more complicated
cases that this approach isn't good enough for, but this
is good enough for now to fix rdar://18067671.
Also make an effort to properly support generic subclasses
of Objective-C classes.
Swift SVN r21506
