Don't permit creating specialized conformances of specialized conformances...

... and stop doing it when we're separating the archetypes of
extensions from the types they extend. I wasn't able to isolate a
useful test case for this; it triggers when building the standard
library with extension archetypes separated.


Swift SVN r20814

lib/AST/Module.cpp

@@ -836,7 +836,7 @@ LookupConformanceResult Module::lookupConformance(Type type,
       explicitConformanceType = nominal->getDeclaredTypeInContext();
     } else {
       explicitConformanceType = cast<ExtensionDecl>(declaresConformance)
-        ->getDeclaredTypeInContext();
+        ->getExtendedType()->getAnyNominal()->getDeclaredTypeInContext();
     }
 
     // If the explicit conformance is associated with a type that is different