Closed requested to merge wip/adinapoli-issue-20081 into master
See #20081 (closed) and #18516 for context.
This work-in-progress MR capitalises on the work started on #19930 (closed) by slowly porting all those TcRn diagnostics currently wrapped into an
UnknownTcRnMessage into proper type constructors for the
TcRnMessage type, together with their hints, if any.