Add @phutil-external-symbol declarations to Phabricator

Summary: See D1381.

Test Plan: Ran "arc liberate src/ --all" and got a clean rebuild.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T762

Differential Revision: https://secure.phabricator.com/D1382