Alfredo Di Napoli authored May 11, 2021 and Marge Bot committed Jun 03, 2021

This commit converts a bunch of HsToCore (Ds) messages to use the new
GHC's diagnostic message infrastructure. In particular the DsMessage
type has been expanded with a lot of type constructors, each
encapsulating a particular error and warning emitted during desugaring.

Due to the fact that levity polymorphism checking can happen both at the
Ds and at the TcRn level, a new `TcLevityCheckDsMessage` constructor has
been added to the `TcRnMessage` type.