R1:495cb7a2e037

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