Java: Modifier.getLocation() does not work

**Description of the issue**
It is not possible to use `Modifier.getLocation()`, apparently because `Modifier` is not a `@sourceline` / `@locatable`.

https://lgtm.com/query shows the following error:
```
@sourceline incompatible with Modifier::Modifier
```

It would really be useful to check where a modifier occurs, e.g. to be able to tell:
- If it occurs in front of an annotation for the same Modifiable
- If the modifiers are in the conventional order