Add callable type to PHP

Summary:
This diff adds a new primitive `callable` type to the PHP language support. This represents anything which can be called as a function in PHP (e.g. a function name as string, a closure, an array with two entries for static class methods, etc.).

Additionally, for return types, when two return types are detected with one being a callable and one an actual function, the return type is set to just a callable.

Reviewers: #kdevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7034