Fix crash during type checking when adding members to an extension.

If the extension ended up with ErrorType, we could crash while we
continue trying to process its body adding members.

Just bail out in this case.

rdar://problem/27671033

Author: rudkx

lib/AST/NameLookup.cpp

@@ -898,6 +898,9 @@ void NominalTypeDecl::addedMember(Decl *member) {
 
 void ExtensionDecl::addedMember(Decl *member) {
   if (NextExtension.getInt()) {
+    if (getExtendedType()->is<ErrorType>())
+      return;
+
     auto nominal = getExtendedType()->getAnyNominal();
     if (nominal->LookupTable.getPointer()) {
       // Make sure we have the complete list of extensions.