We defend against a direct cycle where a base class ValueObject is its
own parent, but not against a longer base cycle. This cycle requires
that some value's Type includes a base class, and that base class is in
a class hierarchy that cycles back to the original base class.
I wrote a test case that creates a cycle in the class hierarchy by
dynamically overwriting the superclass of an object, but I can't
reproduce the crash. I can't think of any other way to make a real
object that behaves that way. Maybe is a type system problem in making
up the type for whatever type we're trying to ingest here.
While unsatisfying, without a reproducer this is the best we can do for
now.
rdar://140293233
b171ebbfff