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