[pfff/php] add support for foreach ($xs await ...)

Summary: @override-unit-failures

Test Plan:
unit test included
make tests

Reviewers: jcreed, jwatzman, eletuchy, glevi, julienv, pieter

Reviewed By: pieter

Subscribers: jwilson

Differential Revision: https://phabricator.fb.com/D1473330

Tasks: 4811024
Merge branch 'master' of github.com:facebook/pfff