Fix IsConcreteType for generic classes and add IsType predicate. (#1204)

* check that the return type is a type, add more implicit conversions

* update IsConcreteType for generic classes, add IsType predicate

* delete a comment

* trivial change

* move Witness wrt IsConcreteType and IsType

* added comments to tests

* update error line numbers

executable_semantics/interpreter/value.cpp

@@ -361,9 +361,7 @@ void Value::Print(llvm::raw_ostream& out) const {
       out << "\"";
       break;
     case Value::Kind::TypeOfClassType:
-      out << "typeof("
-          << cast<TypeOfClassType>(*this).class_type().declaration().name()
-          << ")";
+      out << "typeof(" << cast<TypeOfClassType>(*this).class_type() << ")";
       break;
     case Value::Kind::TypeOfInterfaceType:
       out << "typeof("