[iter] Use aliasing using for types

Fix ambiguity of hb_sorted_array_t::item_t with gcc.  No idea if that's a gcc bug
or what spec requires, but using aliasing using seems to fix it.  It probably breaks
our non-C++11 bots, in which case I have to condition the change.  Testing.

src/hb-iter.hh

@@ -94,8 +94,8 @@ struct hb_iter_t
 };
 
 #define HB_ITER_USING(Name) \
-  using typename Name::iter_t; \
-  using typename Name::item_t; \
+  using iter_t = typename Name::iter_t; \
+  using item_t = typename Name::item_t; \
   using Name::item_size; \
   using Name::is_iterator; \
   using Name::iter; \