If the substitution terms of a concrete type symbol contain unresolved
name symbols, we have an invalid requirement that is dropped from the
minimized signature. In this case, the rewrite system used for minimization
cannot be installed as the official rewrite system for this generic
signature, because building a rewrite system from the signature will
produce a different result.
This might be the cause of the crash in rdar://114111159.
595a7a530b