Let arc liberate throw on unsupported PHP features

Summary: Ref T4725.

Test Plan: add some files that have unsupported constructs all over the place and run arc liberate

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4725

Differential Revision: https://secure.phabricator.com/D9585