Convert more TcRnUnknownMessage into named constructors in TcRnMessage
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.
Edited by Alfredo Di Napoli