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.
* [exec.snd.expos]p39 Unable to find paragraph to replace.
  Referenced paragraph starts:
    "Let Sndr be a (possibly const-qualified) specialization basic-sender"
  Inserted changes as new text after [exec.snd.expos]p41 instead.

Also fixes LWG4203.

Author: burblebee