We can implement the trivial {default,copy,move} construction outselves
inside qmetatype.cpp and qvariant.cpp, simplifying the QMetaType
interface object, removing up to three relocations per QMTI.
This adds the testing for QMetaType::isXxxConstructible and
isDestructible that couldn't be added before.
Change-Id: Ic44396b31ba04712aab3fffd16ff0a28f541d507
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>