[APIView] Support adding comments to removed lines

At the time of writing this issue, ApiView doesn't support adding comments to removed lines of code in a diff. It's a valid case to do so if there are questions on why the API was removed (as this is a major breaking change).

It'd be nice to support this rather than the commenter needing to add the comment to an added or unchanged line and saying the comment is actually for a different line in the ApiView diff.