Add support for anonymous class

Summary: Ref T4334. Adds support for anonymous class, which is supported by PHP 7.0.

Test Plan: Added test cases.

Reviewers: joshuaspence, Blessed Reviewers, thaiphv, epriestley

Reviewed By: joshuaspence, Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley, Firehed

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D19736