Show lint messages in deleted files on the left-hand side of the change

Summary:
See PHI416. If you raise a lint message in a deleted file, we don't render any text on the right hand side so the message never displays.

This is occasionally still legitimate/useful, e.g. to display a "don't delete this file" message. At least for now, show these messages on the left.

Test Plan: Posted a lint message on a deleted file via harbormaster.sendmessage, viewed revision, saw file expand with synthetic inline for lint.

Differential Revision: https://secure.phabricator.com/D19171