C++: Add support for getting referenced literals in using declarations

Author: IdrissRio

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

@@ -174,7 +174,26 @@ class UsingDeclarationEntry extends UsingEntry {
    */
   Declaration getDeclaration() { usings(underlyingElement(this), unresolveElement(result), _, _) }
 
-  override string toString() { result = "using " + this.getDeclaration().getDescription() }
+  /**
+   * Gets the member that is referenced by this using declaration, where the member depends on a
+   * type template parameter.
+   *
+   * For example:
+   * ```
+   * template <typename T>
+   * class A {
+   *   using T::m;
+   * };
+   * ```
+   * Here, `getReferencedMember()` yields the member `m` of `T`.
+   * Observe that, as `T` is not instantiated, `m` is represented by a `Literal` and not a `Declaration`.
+   */
+  Literal getReferencedMember() { usings(underlyingElement(this), unresolveElement(result), _, _) }
+
+  override string toString() {
+    result = "using " + this.getDeclaration().getDescription() or
+    result = "using " + this.getReferencedMember()
+  }
 }
 
 /**