Mark return types as uses for PHP language

Summary:
Return types (when they are an object) are now marked as uses.

Note that the return type uses of closures do not actually work. This also happens for normal parameters, so I think this is a separate issue, I'll look into that later.

Reviewers: KDevelop

Subscribers: kdevelop-devel, KDevelop

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D6675