Better support iterable typehint in PHP language

Summary: This diff adds type support for `iterable` (which is either an object implementing `Traversable` or an `array`).

Reviewers: #kdevelop, zhigalin, pprkut

Reviewed By: zhigalin

Subscribers: zhigalin, kdevelop-devel, #kdevelop

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D6644