Both QStringView and QAnyStringView implicitly convert from any
container with a fitting value_type, and working std::data, std::size,
std::begin and std::end.
Add these missing operations (and complementary ones) to XmlStringRef,
so it implicitly converts to QStringView and QAnyStringView, too.
Add a check to that effect and remove the now-superfluous operator
QStringView().
Task-number: QTBUG-103302
Change-Id: I89d586cf64447a82022e06d546d7ee8339fc6dc7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
39473f9210