QString::arg(char16_t{}) prefers the integral, not the QChar overload

QString fmt = "%1";
assert(fmt.arg(u' ') == " "); // FAIL: result is "32"

Problem here is that we have a large historic body of code which may rely on this. Maybe we need to = delete the overload?

The QLatin1StringView and QStringView overloads use the multi-arg code path for this, and treat char16_t as a character.

This also means that the behaviour differs between QString::arg() and QString::multiArg().