C++: Fix join-order problem in cpp/command-line-injection

Before on Abseil Linux:
```
Evaluated relational algebra for predicate ExecTainted::ExecState#class#91000ffb#fff@41084cm7 with tuple counts:
        40879811  ~0%    {2} r1 = SCAN DataFlowUtil::Node::getLocation#dispred#f0820431#ff OUTPUT In.1, In.0
        40879811  ~0%    {2} r2 = JOIN r1 WITH Location::Location::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1
            7527  ~3%    {3} r3 = JOIN r2 WITH ExecTainted::interestingConcatenation#91000ffb#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0
            7527  ~0%    {4} r4 = JOIN r3 WITH DataFlowUtil::Node::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.0, Rhs.1
            7527  ~0%    {5} r5 = JOIN r4 WITH DataFlowUtil::Node::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.0, Lhs.3, Rhs.1
            7527  ~0%    {6} r6 = JOIN r5 WITH DataFlowUtil::Node::getLocation#dispred#f0820431#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.0, Lhs.3, Lhs.4
            7527  ~0%    {3} r7 = JOIN r6 WITH Location::Location::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT ((((((("ExecState (" ++ Rhs.1) ++ " | ") ++ Lhs.4) ++ ", ") ++ Lhs.1) ++ " | ") ++ Lhs.5 ++ ")"), Lhs.3, Lhs.2
                         return r7
```

After:
```
Evaluated relational algebra for predicate ExecTainted::ExecState#class#91000ffb#fff@1ffe61ps with tuple counts:
        7527  ~0%    {3} r1 = JOIN ExecTainted::interestingConcatenation#91000ffb#ff WITH DataFlowUtil::Node::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Rhs.1
        7527  ~0%    {4} r2 = JOIN r1 WITH DataFlowUtil::Node::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.0, Lhs.1, Lhs.2, Rhs.1
        7527  ~1%    {5} r3 = JOIN r2 WITH DataFlowUtil::Node::getLocation#dispred#f0820431#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0, Lhs.2, Lhs.3
        7527  ~0%    {5} r4 = JOIN r3 WITH Location::Location::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.4, Rhs.1
        7527  ~4%    {6} r5 = JOIN r4 WITH DataFlowUtil::Node::getLocation#dispred#f0820431#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.0, Lhs.1, Lhs.2, Lhs.3, Lhs.4
        7527  ~0%    {3} r6 = JOIN r5 WITH Location::Location::toString#dispred#f0820431#ff ON FIRST 1 OUTPUT ((((((("ExecState (" ++ Rhs.1) ++ " | ") ++ Lhs.3) ++ ", ") ++ Lhs.5) ++ " | ") ++ Lhs.4 ++ ")"), Lhs.1, Lhs.2
                     return r6
```

Author: jketema

cpp/ql/src/Security/CWE/CWE-078/ExecTainted.ql

@@ -77,7 +77,7 @@ class ExecState extends DataFlow::FlowState {
   ExecState() {
     this =
       "ExecState (" + fst.getLocation() + " | " + fst + ", " + snd.getLocation() + " | " + snd + ")" and
-    interestingConcatenation(fst, snd)
+    interestingConcatenation(pragma[only_bind_into](fst), pragma[only_bind_into](snd))
   }
 
   DataFlow::Node getFstNode() { result = fst }