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