Summary:
QT_USE_QSTRINGBUILDER is the official successor for
QT_USE_FAST_OPERATOR_PLUS & QT_USE_FAST_CONCATENATION, extending coverage to QByteArray.

Test Plan: Builds, local tests fail as before, used for few days without issues.

Reviewers: #kdevelop, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7687