Rawtypes warnings should be produced for pattern matching in instanceof

Based on:
https://github.com/openjdk/jdk/pull/2913/files/f7924d278d3faf6b7a13a14898e22419bf6bcc04#r594176266

Consider code like:
```
        Object o = null;
        boolean b = o instanceof List l;
```

This should produce a rawtype warning for List, but it does not.