`with-await-transform::await_transform` should not use a deduced return type

[t3nsor]

https://eel.is/c++draft/exec.awaitable#5

The use of the deduced return type causes the definition of the sender's as_awaitable method to be instantiated too early, e.g., when the sender is passed to get_completion_signatures.

[ericniebler]

Proposed resolution:

Change [exec.awaitable] p5 as follows:

5. Let with-await-transform be the exposition-only class template:

   namespace std::execution {
     template<class T, class Promise>
       concept has-as-awaitable =                                  // exposition only
         requires (T&& t, Promise& p) {
           { std::forward<T>(t).as_awaitable(p) } -> is-awaitable<Promise&>;
         };
   
     template<class Derived>
       struct with-await-transform {                               // exposition only
         template<class T>
           T&& await_transform(T&& value) noexcept {
             return std::forward<T>(value);
           }
   
         template<has-as-awaitable<Derived> T>
   -       decltype(auto) await_transform(T&& value)
   -         noexcept(noexcept(std::forward<T>(value).as_awaitable(declval<Derived&>()))) {
   +       auto await_transform(T&& value)
   +         noexcept(noexcept(std::forward<T>(value).as_awaitable(declval<Derived&>())))
   +         -> decltype(std::forward<T>(value).as_awaitable(declval<Derived&>())) {
             return std::forward<T>(value).as_awaitable(static_cast<Derived&>(*this));
           }
       };
   }

[jwakely]

Created https://cplusplus.github.io/LWG/issue4201