QStringView-related API missing in Qt5

Some QStringView related API is missing in Qt5, compared to what Qt6 has, related to easier porting from QStringRef to QStringView:

• QStringView::mid / left / right / chop / truncate have undefined behavior or assert when corresponding QStringRef / QString methods return an empty ref / whole string, or similar
• QStringView::split is missing
• QStringView comparison with const char[] is not working, requires wrapping in QLatin1String or similar
• QStringView::toInt(...) et al missing, requiring to convert to QString first
• QStringView::count(...) missing
• QRegularExpression::(global)match(QStringView...) is missing
• QString::operator+=(QStringView) / append / prepend / insert missing
• QString::localeAwareCompare(QStringView), count missing