Make XHPAST keep unterminated mutiline comments instead of discarding them

Summary: We don't currently emit a token when we hit EOF if in the comment or
doc comment state. In these states, emit a token on EOF.

Test Plan: Ran XHPAST against inputs like '<?php /* x' and '<?php /** x'.

Reviewers: nh, btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T812

Differential Revision: https://secure.phabricator.com/D1530