Use underlyingElement in isStructuredBinding

Accodring to the documentation in `Element.qll`, `underlyingElement` is
supposed to be used here and not `unresolveElement`.

Author: jketema

cpp/ql/lib/semmle/code/cpp/Variable.qll

@@ -173,7 +173,7 @@ class Variable extends Declaration, @variable {
    * Holds if this variable is declated as part of a structured binding
    * declaration. For example, `x` in `auto [x, y] = ...`.
    */
-  predicate isStructuredBinding() { is_structured_binding(unresolveElement(this)) }
+  predicate isStructuredBinding() { is_structured_binding(underlyingElement(this)) }
 
   /**
    * Holds if this is a compiler-generated variable. For example, a