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