Do not enforce TypePtr when calling AbstractType::indexed

When the type is _not_ in a TypePtr, but e.g. handled via a
unique_ptr, then this would potentially lead to double frees:

```
auto type = std::make_unique<SomeKDevelopType>();
type->indexed(); // creates TypePtr, destroys it, deletes the type
// type now holds on to a dangling pointer
// dtor of type would try to delete again, and crash
```