[Flight] Allow Temporary References to be awaited (#34084)

Fixes #33534.

`.then` method can be tested when you await a value that's not a
Promise. For regular Client References we have a way to mark those as
"async" and yield a reference to the unwrapped value in case it's a
Promise on the Client.

However, the realization is that we never serialize Promises as opaque
when passed from the client to the server. If a Promise is passed, then
it would've been deserialized as a Promise (while still registered as a
temporary reference) and not one of these Proxy objects.

Technically it could be a non-function value on the client which would
be wrong but you're not supposed to dot into it in the first place.

So we can just assume it's `undefined`.