`when_all`'s receiver's environment is incorrectly forwarding all queries

[ericniebler]

[exec.when.all] p6 reads:

6. The member impls-for<when_all_t>::get-env is initialized with a callable object
   equivalent to the following lambda expression:

   []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
     return see below;
   }

   Returns an object e such that

   1. decltype(e) models queryable, and

   2. e.query(get_stop_token) is expression-equivalent to state.stop-src.get_token(), and

   3. given a query object q with type other than cv stop_token_t, e.query(q) is
      expression-equivalent to get_env(rcvr).query(q).

The problem is in (6.3). It should be forwarding on forwarding-query's to get_env(rcvr) but it is instead forwarding all queries.

Proposed resolution:

Change [exec.snd.expos] p2 as follows (underlined text is new):

2. For a queryable object env, FWD-ENV(env) is an expression whose
   type satisfies queryable such that for a query object q and a pack
   of subexpressions as, the expression FWD-ENV(env).query(q, as...)
   is ill-formed if forwarding_query(q) is false; otherwise, it is
   expression-equivalent to env.query(q, as...).
   The type FWD-ENV-T(Env) is decltype(FWD-ENV(declval<Env>())).

Change [exec.when.all] para 6 as follows:

3. given a query object q with type other than cv stop_token_tand whose type satisfies
   forwarding-query, e.query(q) is expression-equivalent to get_env(rcvr).query(q).

Change the definition of make-state in [exec.when.all] para 7 as follows:

 template<class Rcvr>
 struct make-state {
-  template<max-1-sender-in<env_of_t<Rcvr>>... Sndrs>
+  template<max-1-sender-in<FWD-ENV-T(env_of_t<Rcvr>)>... Sndrs>

Change para 9 as follows:

9. The alias values_tuple denotes the type

   -tuple<value_types_of_t<Sndrs, env_of_t<Rcvr>, decayed-tuple, optional>...>
   +tuple<value_types_of_t<Sndrs, FWD-ENV-T(env_of_t<Rcvr>), decayed-tuple, optional>...>

   if that type is well-formed; otherwise, tuple<>.