Fix class attribute node in anonymous class
Summary: Ref T4334. The class attribute node of an anonymous class should be n_EMPTY.
Test Plan:
Tested with phabricator xhpast view
Reviewers: joshuaspence, Blessed Reviewers, epriestley
Reviewed By: joshuaspence, Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley, Firehed
Maniphest Tasks: T4334
Differential Revision: https://secure.phabricator.com/D19740