[GSB] Use precise potential-archetype identity when forming protocol-requirement sources.

When we’re determining what type should be used to key a ProtocolRequirement based on the source’s potential archetype and the target’s potential archetype, use potential archetype identity (==) rather than equivalence-class membership to cover the basis case. This ensures that interesting identity relationships in the enclosing context (e.g., the current GenericSignatureBuilder) don’t cause us to compute incorrect stored types.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -803,7 +803,7 @@ static Type formProtocolRelativeType(ProtocolDecl *proto,
                                      PotentialArchetype *basePA,
                                      PotentialArchetype *pa) {
   // Basis case: we've hit the base potential archetype.
-  if (basePA->isInSameEquivalenceClassAs(pa))
+  if (basePA == pa)
     return proto->getSelfInterfaceType();
 
   // Recursive case: form a dependent member type.