[GSB] Clean up and audit uses of ArchetypeResolutionKind.

Use ArchetypeResolutionKind::CompleteWellFormed whenever we need to
ask questions about the potential archetype, and
ArchetypeResolutionKind::WellFormed when we need only evaluate whether
there is a legitimate type with that name (and possibly get a handle
to it).

lib/AST/GenericSignatureBuilder.cpp

@@ -1960,14 +1960,10 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
     }
 
     case ArchetypeResolutionKind::AlreadyKnown:
-      break;
+        return nullptr;
     }
   }
 
-  // If we still don't have a result potential archetype, we're done.
-  if (!resultPA)
-    return nullptr;
-
   // If we have a potential archetype that requires more processing, do so now.
   if (shouldUpdatePA) {
     // For concrete types, introduce a same-type requirement to the aliased