C++: Add support for C++ using aliases

Previously these were identified as typedefs.

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -675,7 +675,7 @@ decltypes(
   | 2 = class
   | 3 = union
   | 4 = enum
-  | 5 = typedef
+  | 5 = typedef                       // classic C: typedef typedef type name
   | 6 = template
   | 7 = template_parameter
   | 8 = template_template_parameter
@@ -684,6 +684,7 @@ decltypes(
 // ... 11 objc_protocol deprecated
 // ... 12 objc_category deprecated
   | 13 = scoped_enum
+  | 14 = using_alias                  // a using name = type style typedef
   ;
 */
 usertypes(