`@name` on top-level type silently does nothing

Right now if you do something like

```cddl
out_ref = _CDDL_CODEGEN_EXTERN_TYPE_ ; @name Foo
```

The `@name` notation will silently do nothing (it only works for giving names to things inside structs). We don't need to support this, but if we don't we should probably throw an error instead of silently doing nothing