When forming the default witness table for a resilient protocol, look
for default associated conformances as well. These are identified by
associated conformance requirements whose root associated type has a
default type. For such requirements, we look for a conformance and
record it as a default associated conformance.
Emit a warning in cases where we don't find such a conformance,
because the associated type and conformance *may* have been added
with the intent of being resilient, and we can't know. This warning
might be a terrible idea, but it is only enabled under
-enable-resilience (which itself is hidden) and fires in one only
place in the standard library (which seems legitimate), so we'll try
it for now.
