Generate code that uses nullptr & override

Test Plan:
kdevelop qmake* plugins and kdev-php plugins build fine with it (and no more
have warnings from -Wsuggest-override -Wzero-as-null-pointer-constant.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D13938