Add support for saving/restoring async stack frames to default co_viaIfAsync() implementation

Summary:
This should avoid creating two temporary wrapper coroutines to apply
both co_viaIfAsync() and co_withAsyncStack() to awaitables when awaited
within a Task/AsyncGenerator coroutine.

The default co_viaIfAsync() implementation now customises the
co_withAsyncStack() CPO and saves and restores the async frame
itself if the wrapped awaitable does not support async stacks,
or otherwise passes the async frame through to the child awaitable
if it does support async stacks.

Reviewed By: andriigrynenko

Differential Revision: D24464513

fbshipit-source-id: f9751030f3c1d2725a00cb803ee46ee12dddb2ed

folly/experimental/coro/WithAsyncStack.h

@@ -206,6 +206,10 @@ struct WithAsyncStackFunction {
 
 } // namespace detail
 
+template <typename Awaitable>
+inline constexpr bool is_awaitable_async_stack_aware_v =
+    folly::is_tag_invocable_v<detail::WithAsyncStackFunction, Awaitable>;
+
 // Coroutines that support the AsyncStack protocol will apply the
 // co_withAsyncStack() customisation-point to an awaitable inside its
 // await_transform() to ensure that the current coroutine's AsyncStackFrame