fix issue with parsing optional inside nested type arg

Summary:
There was no production for

   <?hh
   class Foo {
     public Map<int, ?Vector<int>> $x;
   }

Test Plan: manual; make test

Reviewers: pad

Reviewed By: pad

CC: amenghra

Differential Revision: https://phabricator.fb.com/D735487

Task ID: 1859587