Add some easy TcRn messages
Me and @rae might need to discuss how to tame the zoo of logging functions we have inside
GHC.Tc.Utils.Monad: at the moment I have added two provisional functions,
addTcRnDiagnostic to avoid creating another big MR, but obviously we want to use very few functions (ideally one or two). Suggestions about the best migration path welcome.
Surprisingly enough, we didn't have a test for the dodgy exports flag, so I added it.