guard against self-exchange in the get_completion_scheduler query

[ericniebler]

in get_completion_scheduler_t, we have the following:

__prev = std::exchange(__sch, __read_query_t{}(__sch, __env...));

when __sch.query(get_completion_scheduler) returns a reference to *this, the code above is essentially doing std::exchange(__sch, __sch). std::exchange does:

template <class T, class U = T>
T exchange(T& old_value, U&& new_value) {
  auto old = std::move(old_value);
  old_value = std::forward<U>(new_value);
  return old;
}

when &old_value == &new_value, the first line puts new_value into a moved-from state. then we assign that moved-from object to old_value, which is UB in most cases, and never what you want to happen.