Use return typehints in typebuilder for PHP language
Summary: The return typehints are now used for function signatures. I also inlined TypeBuilder::openFunctionType into TypeBuilder::visitClassStatement since it's only used there and it makes the code clearer in my opinion.
Subscribers: brauch, kdevelop-devel, KDevelop
Differential Revision: https://phabricator.kde.org/D6680