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