Fix an XHPAST bug
Summary: Currently, XHPAST fails to correctly parse myfunc($var1, &$var2);.
Test Plan:
Before
{F261487}
After
{F261488}
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11216