co_composed.hpp: Guard use of async_operation

`async_operation` is defined only when concepts are in use.
When ASIO_DISABLE_CONCEPTS is defined, bare `typename`
should be used instead.

We can't use ASIO_ASYNC_OPERATION macros here, as it currently
does not have a no-argument form.

Fixes: chriskohlhoff#1547

asio/include/asio/co_composed.hpp

@@ -845,7 +845,11 @@ class co_composed_promise
     throw;
   }
 
+#ifdef ASIO_DISABLE_CONCEPTS
+  template <typename Op>
+#else
   template <async_operation Op>
+#endif
   auto await_transform(Op&& op
 #if defined(ASIO_ENABLE_HANDLER_TRACKING)
 # if defined(ASIO_HAS_SOURCE_LOCATION)