Mark PhabricatorPHIDType::getPHIDTypeApplicationClass() as abstract
Summary: Fixes T9625. As explained in a TODO comment, seems reasonable enough.
Test Plan: Unit tests.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, hach-que
Maniphest Tasks: T9625
Differential Revision: https://secure.phabricator.com/D14068