Generated IPC serializers forMoveOnlyBaseClass&& are incorrect

It appears that the code tries to implement move through base class rvalue reference. It is not possible in general case following c++ rvalue semantics, as rvalue constructors are not polymorphic.

Move makes no sense:

void ArgumentCoder<WebCore::MoveOnlyBaseClass>::encode(Encoder& encoder, WebCore::MoveOnlyBaseClass&& instance)
{
    if (auto* subclass = dynamicDowncast<WebCore::MoveOnlyDerivedClass>(instance)) {
        encoder << WebCore_MoveOnlyBaseClass_Subclass::MoveOnlyDerivedClass;
        encoder << WTFMove(*subclass);
        return;
    }
    ASSERT_NOT_REACHED();
}