AST: Don't build archetypes with an error type as the superclass

This breaks invariants.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -2020,11 +2020,13 @@ Type GenericSignatureBuilder::PotentialArchetype::getTypeInContext(
   Type superclass = representative->getSuperclass();
   if (superclass && superclass->hasTypeParameter()) {
     if (representative->RecursiveSuperclassType) {
-      superclass = ErrorType::get(superclass);
+      superclass = Type();
     } else {
       superclass = genericEnv->mapTypeIntoContext(
                                               superclass,
                                               builder.getLookupConformanceFn());
+      if (superclass->is<ErrorType>())
+        superclass = Type();
 
       // We might have recursively recorded the archetype; if so, return early.
       // FIXME: This should be detectable before we end up building archetypes.

lib/AST/Type.cpp

@@ -2558,6 +2558,8 @@ CanArchetypeType ArchetypeType::getNew(
                                    SmallVectorImpl<ProtocolDecl *> &ConformsTo,
                                    Type Superclass,
                                    LayoutConstraint Layout) {
+  assert(!Superclass || Superclass->getClassOrBoundGenericClass());
+
   // Gather the set of protocol declarations to which this archetype conforms.
   ProtocolType::canonicalizeProtocols(ConformsTo);
 
@@ -2578,6 +2580,7 @@ ArchetypeType::getNew(const ASTContext &Ctx,
                       SmallVectorImpl<ProtocolDecl *> &ConformsTo,
                       Type Superclass,
                       LayoutConstraint Layout) {
+  assert(!Superclass || Superclass->getClassOrBoundGenericClass());
   assert(genericEnvironment && "missing generic environment for archetype");
 
   // Gather the set of protocol declarations to which this archetype conforms.