`ApplyTypeAnnotationsVisitor` does not respect `overwrite_existing_annotations` for attributes

Consider this example:

```py
class X:
x: int
```

used to patch:

```py
class X:
x: str
```

`ApplyTypeAnnotationsVisitor` will *not* change the type annotation on `X.x` to `int` even if you set `overwrite_existing_annotations` to True. This is the case even if you change it to be an assign (as in `x: str = "a"`.)