Statement encloses itself

**Description of the issue**
It is possible for a `Stmt` to enclose itself.
Is this expected behavior? If yes, maybe add a note to the docs?

```codeql
import java

from Stmt s
where s.getEnclosingStmt() = s
select s
```
Link to query:
https://lgtm.com/query/4871075667428522469/