[meta] Remove hb_add_const

src/hb-iter.hh

@@ -289,7 +289,7 @@ struct hb_is_source_of
 {
   private:
   template <typename Iter2 = Iter,
-	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, hb_add_lvalue_reference<hb_add_const<Item>>))>
+	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, hb_add_lvalue_reference<const Item>))>
   static hb_true_type impl (hb_priority<2>);
   template <typename Iter2 = Iter>
   static auto impl (hb_priority<1>) -> decltype (hb_declval (Iter2) >> hb_declval (Item &), hb_true_type ());

src/hb-meta.hh

@@ -107,7 +107,6 @@ template <typename T> static inline T hb_declval ();
 template <typename T> struct hb_match_const		: hb_type_identity_t<T>, hb_false_type	{};
 template <typename T> struct hb_match_const<const T>	: hb_type_identity_t<T>, hb_true_type	{};
 template <typename T> using hb_remove_const = typename hb_match_const<T>::type;
-template <typename T> using hb_add_const = const T;
 
 template <typename T> struct hb_match_reference		: hb_type_identity_t<T>, hb_false_type	{};
 template <typename T> struct hb_match_reference<T &>	: hb_type_identity_t<T>, hb_true_type	{};