C++: Fix join-order problem in getNextSwitchCase

Before on `neovim`:
```
[2026-04-28 14:54:20] Evaluated non-recursive predicate Stmt::SwitchCase.getNextSwitchCase/0#dispred#2d3cb6d3@ac8178o2 in 68ms (size: 20848).
Evaluated relational algebra for predicate Stmt::SwitchCase.getNextSwitchCase/0#dispred#2d3cb6d3@ac8178o2 with tuple counts:
           21888  ~0%    {2} r1 = SCAN switch_case OUTPUT In.2, In.0
           21888  ~0%    {4}    | JOIN WITH #switch_caseMerge_21#join_rhs ON FIRST 1 OUTPUT Lhs.1, Lhs.0, _, Rhs.1
           21888  ~4%    {3}    | REWRITE WITH Tmp.2 := 1, Out.2 := (In.3 - Tmp.2) KEEPING 3
        24091916  ~0%    {3}    | JOIN WITH switch_case ON FIRST 1 OUTPUT Lhs.2, Rhs.2, Lhs.1
           20848  ~2%    {2}    | JOIN WITH #switch_caseMerge_12#join_rhs ON FIRST 2 OUTPUT Lhs.1, Lhs.2
                         return r1
```

After:
```
[2026-04-28 15:30:53] Evaluated non-recursive predicate Stmt::SwitchCase.getNextSwitchCase/0#dispred#2d3cb6d3@bf9801oj in 0ms (size: 20848).
Evaluated relational algebra for predicate Stmt::SwitchCase.getNextSwitchCase/0#dispred#2d3cb6d3@bf9801oj with tuple counts:
        21888  ~0%    {4} r1 = SCAN switch_case OUTPUT In.0, _, In.2, In.1
        21888  ~1%    {3}    | REWRITE WITH Tmp.1 := 1, Out.1 := (In.3 + Tmp.1) KEEPING 3
        20848  ~2%    {2}    | JOIN WITH switch_case ON FIRST 2 OUTPUT Lhs.2, Rhs.2
                      return r1
```

Author: jketema

cpp/ql/lib/semmle/code/cpp/stmts/Stmt.qll

@@ -1516,8 +1516,10 @@ class SwitchCase extends Stmt, @stmt_switch_case {
    * which has result `default:`, which has no result.
    */
   SwitchCase getNextSwitchCase() {
-    result.getSwitchStmt() = this.getSwitchStmt() and
-    result.getChildNum() = this.getChildNum() + 1
+    exists(SwitchStmt s, int n |
+      this = s.getSwitchCase(n) and
+      result = s.getSwitchCase(n + 1)
+    )
   }
 
   /**