Fix an XHPAST bug

Summary: Currently, XHPAST fails to correctly parse myfunc($var1, &$var2);.

Test Plan:
Before

After

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11216