GSB: Don't compute anchors in resolveDependentMemberTypes()

When a concrete type contains unresolved DependentMemberTypes, we
don't need to replace them with their anchor, just the resolved
representative from the PotentialArchetype.

This means that the only time we compute anchors are when
computing canonical types (which is only done after a GSB has
been finalized) and when adding same-type requirements.

This should eventually enable building the same-type rewrite
system in one shot.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -3777,12 +3777,10 @@ static Type resolveDependentMemberTypes(
       return concreteType;
 
     // Map the type parameter to an equivalence class.
-    auto equivClass = resolved.getEquivalenceClass(builder);
-    if (!equivClass)
-      return ErrorType::get(Type(type));
+    auto equivClass = resolved.getEquivalenceClassIfPresent();
 
     // If there is a concrete type in this equivalence class, use that.
-    if (equivClass->concreteType) {
+    if (equivClass && equivClass->concreteType) {
       // .. unless it's recursive.
       if (equivClass->recursiveConcreteType)
         return ErrorType::get(Type(type));
@@ -3797,7 +3795,7 @@ static Type resolveDependentMemberTypes(
                                          resolutionKind);
     }
 
-    return equivClass->getAnchor(builder, builder.getGenericParams());
+    return resolved.getDependentType(builder);
   });
 }