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