A QVariant(QString) was not convertible to an enum not registered with
Q_ENUM() which worked fine in Qt5.
The same problem exists for QVariant(enum) to QString.
Fix it by not bailing out when no metatype for the enum was found and
try to convert it to a qlonglong instead (which is then correctly
converted to the enum type).
Fixes: QTBUG-109744
Pick-to: 6.5 6.4
Change-Id: Ie7bb016a860455b69508f0f46b36474c9c294f3a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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