The solver's simplifyType() can produce a DependentMemberType with a
concrete or archetype base, if that associated type could not be
resolved during associated type inference.
mapTypeOutOfContext() could assert when given a type that violates
invariants in this manner. Instead, let's directly call subst() here,
providing a conformance lookup function (which will probably end up
returning ErrorTypes).
Fixes https://bugs.swift.org/browse/SR-14672 / rdar://problem/78626943.
