Fix XHPAST parsing of casts

Summary: Fixes T7801.

Test Plan: Went to http://phabricator.local/xhpast/ and inspected the parse tree for $x = (double) $y.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7801

Differential Revision: https://secure.phabricator.com/D12344