Fix for optional types in constraints in pfff parser for php/hack

Summary:
Allows only for a single nullable prefix.
This doesn't fix multi nullable, e.g. ???int in constraints

Test Plan: ./pfff -dump_php tests/php/parsing/generics_constraint.php

Reviewers: glevi, amenghra, pad

Reviewed By: glevi

CC: joelm

Differential Revision: https://phabricator.fb.com/D963483

Task ID: 2443278