pfff: support the   <T as XAble> syntax for constraints on generics

Test Plan:
unit test included
make test

Reviewers: julienv, pieter, zoel, amenghra

Reviewed By: pieter

CC: ide

Differential Revision: https://phabricator.fb.com/D769821

Task ID: 1908869
* tests/php/parsing/generics_constraint.php: