Use unsatisfiable for missing methods w/ defaults

When a class instance has an Unsatisfiable constraint in its context
and the user has not explicitly provided an implementation of a method,
we now always provide a RHS of the form `unsatisfiable @msg`, even
if the method has a default definition available. This ensures that,
when deferring type errors, users get the appropriate error message
instead of a possible runtime loop, if class default methods were
defined recursively.

Fixes #23816