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

Author: IdrissRio

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

@@ -174,7 +174,28 @@ class UsingDeclarationEntry extends UsingEntry {
    */
   Declaration getDeclaration() { usings(underlyingElement(this), unresolveElement(result), _, _) }
 
-  override string toString() { result = "using " + this.getDeclaration().getDescription() }
+  /**
+   * Returns the literal referenced by this `using` declaration.
+   * This is particularly relevant for `using` declarations that involve template instantiations.
+   *
+   * For example:
+   * ```
+   * template <typename T>
+   * class A : public T {
+   *   using T::foo; // `foo` is the literal referenced by this using declaration
+   * };
+   * ```
+   */
+  Literal getLiteral() { usings(underlyingElement(this), unresolveElement(result), _, _) }
+
+  override string toString() {
+    if exists(this.getDeclaration())
+    then result = "using " + this.getDeclaration().getQualifiedName()
+    else
+      if exists(this.getLiteral())
+      then result = "using " + this.getLiteral().toString()
+      else result = "using <unknown>"
+  }
 }
 
 /**