Extract Method fromStdVariantImpl() to share the otherwise
more-or-less identical implementation between the two overloads.
Don't use a constrained template version of fromStdVariantImpl() as
fromStdVariant(), because the constraint would have to be very complex
to continue allowing subclasses of std::variant to be passed.
As a drive-by, mark the valueless_by_exception() path Q_UNLIKELY.
[ChangeLog][QtCore][QVariant] Added overload of fromStdVariant()
taking rvalue std::variant<>s.
Fixes: QTBUG-114134
Change-Id: Ia1c7ae93ab421e6689dc9f2d8d0c2295b23cbbf6
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
8983225d3c