A nested type of an archetype type might be concrete, for example, via a
same-type constraint:
extension SomeProtocol where SomeAssoc == Int {
... Self.SomeAssoc ...
}
This can happen in one of two ways; either the EquivalenceClass of the
nested type has a concrete type, or it is "fully concrete" because
there is no equivalence class and maybeResolveEquivalenceClass() returns
a ResolvedType storing the concrete type.
For some reason we didn't handle the second case here.
Fixes https://bugs.swift.org/browse/SR-13519 / rdar://problem/68531679
