warn-unused-ignores does not respect follow-imports=silent

```
$ cat imported.py
a: int = 'foo'

b: int = 3  # type: ignore

$ cat main.py 
import imported

$ mypy --warn-unused-ignores --follow-imports=silent main.py
imported.py:3: note: unused 'type: ignore' comment
```

`follow-imports=silent` correctly silences the type error on line 1 of `imported.py`, but fails to silence the "unused type: ignore comment" note.