P3557R3 High-Quality Sender Diagnostics with Constexpr Exceptions

* [exec.util.cmplsig] Subclause (now without siblings) promoted to \rSec1
  (replacing [exec.util]) and renamed to [exec.cmplsig] "Completion signatures".
* [exec.snd.expos]p24 Added missing \expos comments.

Also fixes LWG4203.

Author: burblebee