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