Add spaceship and null coalesce operators to PHP language support

Summary: This diff adds the spaceship operator `<=>` (returning `-1`, `0` or `1`, useful in implementing comparison functions) and the null coalesce operator `??` (returning the left operand if it is set and is not null, otherwise the second operand).

Reviewers: #kdevelop, zhigalin, pprkut

Reviewed By: zhigalin

Subscribers: zhigalin, kdevelop-devel, #kdevelop

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D6645