C++: Fix join order in 'cpp/double-free'.

[MathiasVP]

This applies the fix from #3352 to cpp/double-free which suffered from the same issue.

Tuple counts before (on cmake):

Evaluated relational algebra for predicate #select@280ab6i3 with tuple counts:
    11204231  ~0%    {4} r1 = SCAN `Element::ElementBase.toString/0#dispred#6e016f86` OUTPUT In.0, _, In.1, _
    11204231  ~0%    {2}    | REWRITE WITH Tmp.1 := "Memory pointed to by '", Tmp.3 := "' may already have been freed by $@.", Out.1 := (Tmp.1 ++ In.2 ++ Tmp.3) KEEPING 2
        1012  ~3%    {2}    | JOIN WITH `project#FlowAfterFree::isFree/4#cb2bae13#4_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, Lhs.1
           5  ~0%    {3}    | JOIN WITH `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::PathNode.getNode/0#dispred#6f786c66_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0
           5  ~0%    {4}    | JOIN WITH `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::flowPath/2#196f9b68_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.0
           5  ~0%    {5}    | JOIN WITH `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::PathNode.getNode/0#dispred#6f786c66` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3, Lhs.0
           5  ~0%    {5}    | JOIN WITH `project#FlowAfterFree::isFree/4#cb2bae13#6` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3, Lhs.4
           5  ~0%    {6}    | JOIN WITH `Element::ElementBase.toString/0#dispred#6e016f86` ON FIRST 1 OUTPUT Lhs.2, Lhs.4, Lhs.3, Lhs.1, Lhs.0, Rhs.1
                     return r1

After:

Evaluated relational algebra for predicate #select#cpe#1235678@e9734f2j with tuple counts:
    5  ~0%    {2} r1 = SCAN `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::flowPath/2#196f9b68` OUTPUT In.1, In.0
    5  ~0%    {3}    | JOIN WITH `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::PathNode.getNode/0#dispred#6f786c66` ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Rhs.1
    5  ~0%    {4}    | JOIN WITH `DataFlowImpl::Impl<DataFlow::DataFlow::GlobalWithState<DoubleFree::DoubleFree::FlowFromFreeConfig>::C>::PathNode.getNode/0#dispred#6f786c66` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.0
    5  ~0%    {4}    | JOIN WITH `project#FlowAfterFree::isFree/4#cb2bae13#6` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3
    5  ~0%    {5}    | JOIN WITH `Element::ElementBase.toString/0#dispred#6e016f86` ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.3, Lhs.0, Rhs.1
    5  ~0%    {6}    | JOIN WITH `project#FlowAfterFree::isFree/4#cb2bae13#4` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0, Lhs.2, Lhs.3, Lhs.4
    5  ~0%    {7}    | JOIN WITH `Element::ElementBase.toString/0#dispred#6e016f86` ON FIRST 1 OUTPUT Lhs.2, Lhs.3, Lhs.1, Lhs.0, Rhs.1, Lhs.4, Lhs.5
            return r1

[jketema]

Does this need DCA?

[MathiasVP]

Does this need DCA?

Probably not, but I'll start one anyway to be on the safe side. If nothing else, we can get a small dopamine boost from seeing the (probably very small) performance boost 🎉