Asymmetric operator+=(QByteArray, QString) when QStringBuilder is in use

Given:

QByteArray ba;
QString s;

then

// build error
ba += s; 

 // build error, unless QStringBuilder is active, and QT_NO_CAST_TO_ASCII is not defined
ba += (s+s);

Either we support the first case (and we guard it by QT_NO_CAST_TO_ASCII), or we shouldn't be supporting the second case either. In which case, there is no longer anything in Qt that uses QT_NO_CAST_TO_ASCII.