Allow protocol to accept non-class types

```c++
struct Interface{};

protocol p(15); // ERROR: "int is not a complete class type"
```

Should this work? I think probably yes since it will be relevant once there is complete operator support.

Start with the protocol construction shown in the issue and trace the code path that rejects int as an incomplete class type. Determine whether non-class types should be accepted and how that relates to the mentioned operator support; the example should compile with behavior covered by an appropriate test.

Written by the indexing model from the issue text.