C++: Join on two columns instead of one.

Before:
```
Evaluated non-recursive predicate TranslatedElement#ea057665::TranslatedElement::getInstructionVariable#1#dispred#fff@146210id in 201548ms (size: 3469729).
Evaluated relational algebra for predicate TranslatedElement#ea057665::TranslatedElement::getInstructionVariable#1#dispred#fff@146210id with tuple counts:
  ...
     1812768   ~3%    {3} r65 = JOIN num#InstructionTag#c9183db3::OnlyInstructionTag#f WITH TranslatedExpr#043317a1::TranslatedNonFieldVariableAccess#ff CARTESIAN PRODUCT OUTPUT Rhs.1, Lhs.0, Rhs.0
     1812767   ~0%    {4} r66 = JOIN r65 WITH Access#8878f617::Access::getTarget#0#dispred#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.0
  3996407117   ~3%    {5} r67 = JOIN r66 WITH TranslatedElement#ea057665::getIRUserVariable#2#fff_102#join_rhs ON FIRST 1 OUTPUT Lhs.3, Rhs.1, Lhs.1, Lhs.2, Rhs.2
     1815194   ~0%    {3} r68 = JOIN r67 WITH TranslatedExpr#043317a1::getEnclosingDeclaration#1#ff ON FIRST 2 OUTPUT Lhs.3, Lhs.2, Lhs.4
  ...
```

After:
```
Evaluated non-recursive predicate TranslatedExpr#043317a1::accessHasEnclosingDeclarationAndVariable#3#fff@665ccb8o in 865ms (size: 2769549).
Evaluated relational algebra for predicate TranslatedExpr#043317a1::accessHasEnclosingDeclarationAndVariable#3#fff@665ccb8o with tuple counts:
        2769549  ~1%    {3} r1 = JOIN Access#8878f617::Access::getTarget#0#dispred#ff WITH TranslatedExpr#043317a1::getEnclosingDeclaration#1#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0
                        return r1
...
Evaluated non-recursive predicate TranslatedElement#ea057665::TranslatedElement::getInstructionVariable#1#dispred#fff@7d4d33to in 805ms (size: 3469729).
Evaluated relational algebra for predicate TranslatedElement#ea057665::TranslatedElement::getInstructionVariable#1#dispred#fff@7d4d33to with tuple counts:
  ...
  1963209   ~0%    {2} r34 = JOIN TranslatedElement#ea057665::getIRUserVariable#2#fff WITH TranslatedExpr#043317a1::accessHasEnclosingDeclarationAndVariable#3#fff ON FIRST 2 OUTPUT Rhs.2, Lhs.2
  1815194   ~2%    {2} r35 = JOIN r34 WITH TranslatedExpr#043317a1::TranslatedNonFieldVariableAccess#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1, Rhs.1
  1815194   ~0%    {3} r36 = JOIN r35 WITH num#InstructionTag#c9183db3::OnlyInstructionTag#f CARTESIAN PRODUCT OUTPUT Lhs.1, Rhs.0, Lhs.0
  ...
```

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/internal/TranslatedExpr.qll

@@ -888,10 +888,21 @@ class TranslatedNonFieldVariableAccess extends TranslatedVariableAccess {
 
   override IRVariable getInstructionVariable(InstructionTag tag) {
     tag = OnlyInstructionTag() and
-    result = getIRUserVariable(getEnclosingDeclaration(expr), expr.getTarget())
+    exists(Declaration d, Variable v |
+      accessHasEnclosingDeclarationAndVariable(d, v, expr) and
+      result = getIRUserVariable(d, v)
+    )
   }
 }
 
+pragma[nomagic]
+private predicate accessHasEnclosingDeclarationAndVariable(
+  Declaration d, Variable v, VariableAccess va
+) {
+  d = getEnclosingDeclaration(va) and
+  v = va.getTarget()
+}
+
 class TranslatedFieldAccess extends TranslatedVariableAccess {
   override FieldAccess expr;