The maybeResolveEquivalenceClass() method can deallocate equivalence
classes, because it calls updateNestedTypeForConformance(), which
calls addSameTypeRequirement().
Therefore, the EquivalenceClass stored inside a ResolvedType could
become invalid across calls to maybeResolveEquivalenceClass().
This was a problem in one place in particular, when adding a new
same-type constraint between two type parameters.
Fix this by not caching the equivalence class of a PotentialArchetype
in the ResolvedType implementation. The only time an equivalence class
is now stored is when returning an unresolved type, which is acted
upon immediately.
Fixes <https://bugs.swift.org/browse/SR-12812>, <rdar://problem/63422600>.
