Allow ghost atoms to be rendered

Summary: Ref T4558. Allow ghost atoms to be rendered in Diviner. This functionality didn't exist previously, but was hinted at by the TODO comments.

Test Plan: Generated Diviner documentation for rARC and then removed a class (before re-generating the documentation). Navigated to the documentation for the removed class and saw "This atom no longer exists".

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Maniphest Tasks: T4558

Differential Revision: https://secure.phabricator.com/D13114