C++: Use interpolation to avoid a bad join order

[MathiasVP]

@nickrolfe highlighted this bad join:

Evaluated relational algebra for predicate #select@4dc5a3gq with tuple counts:
  1125  ~0%    {5} r1 = SCAN globalvariables OUTPUT In.0, _, In.2, _, _
  1125  ~2%    {2}    | REWRITE WITH Tmp.1 := "The variable '", Tmp.3 := "'", Tmp.4 := " is used in this function but may not be initialized when it is called.", Out.1 := (Tmp.1 ++ In.2 ++ Tmp.3 ++ Tmp.4) KEEPING 2
     6  ~0%    {3}    | JOIN WITH `GlobalUseBeforeInit::useFunc/2#81a8e7f2` ON FIRST 1 OUTPUT Lhs.0, Rhs.1, Lhs.1
     1  ~0%    {2}    | JOIN WITH `GlobalUseBeforeInit::uninitialisedBefore/2#8cd27ca7#bf` ON FIRST 2 OUTPUT Lhs.1, Lhs.2
               return r1

This is exactly the same kind of join order issue we've seen in #16089.

This query isn't in any security suite so it doesn't really make sense to run DCA on it

[nickrolfe]

LGTM

[calumgrant]

Do you need single-quotes around the $@?

[MathiasVP]

We could definitely do that to keep the query message close to what it is now. We don't use '$@' anywhere else in any C/C++ queries, though.

[MathiasVP]

I've merged the PR now. If you prefer a single-quotes around the interpolation I'd be happy to open a follow-up PR