C++: Replace FastTC with iteration in ignoreExpr

Before, `ignoreExprAndDescendants` and its related predicates had this
timing on Wireshark.

    #TranslatedElement::getRealParent#ffPlus#swapped ......... 25.7s
    TranslatedElement::ignoreExprAndDescendants#f ............ 16.9s
    TranslatedElement::getRealParent#ff ...................... 7.2s
    TranslatedElement::ignoreExpr#f .......................... 4.8s
    TranslatedElement::ignoreExpr#f#antijoin_rhs ............. 3.2s
    TranslatedElement::getRealParent#ff_10#higher_order_body . 2.2s

After, it looks like this

    TranslatedElement::ignoreExprAndDescendants#f ............ 23.4s (executed 9 times)
    TranslatedElement::getRealParent#ff ...................... 6.3s
    TranslatedElement::ignoreExpr#f#antijoin_rhs ............. 4.8s
    TranslatedElement::ignoreExpr#f .......................... 3.7s
    TranslatedElement::getRealParent#ff_10#join_rhs .......... 2.5s
    project#TranslatedElement::getRealParent#ff .............. 1.3s

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/TranslatedElement.qll

@@ -36,10 +36,8 @@ private Element getRealParent(Expr expr) {
 predicate isIRConstant(Expr expr) { exists(expr.getValue()) }
 
 /**
- * Holds if `expr` and all of its descendants should be ignored for the purposes
- * of IR generation due to some property of `expr` itself. Unlike
- * `ignoreExpr()`, this predicate does not ignore an expression solely because
- * it is a descendant of an ignored element.
+ * Holds if `expr` should be ignored for the purposes of IR generation due to
+ * some property of `expr` or one of its ancestors.
  */
 private predicate ignoreExprAndDescendants(Expr expr) {
   // Ignore parentless expressions
@@ -59,7 +57,8 @@ private predicate ignoreExprAndDescendants(Expr expr) {
     // REVIEW: Ignore initializers for `NewArrayExpr` until we determine how to
     // represent them.
     newExpr.getInitializer().getFullyConverted() = expr
-  )
+  ) or
+  ignoreExprAndDescendants(getRealParent(expr)) // recursive case
 }
 
 /**
@@ -80,7 +79,7 @@ private predicate ignoreExprOnly(Expr expr) {
  */
 private predicate ignoreExpr(Expr expr) {
   ignoreExprOnly(expr) or
-  ignoreExprAndDescendants(getRealParent*(expr))
+  ignoreExprAndDescendants(expr)
 }
 
 /**