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MessageClass split and partial refactoring of GHC.Tc.Errors

Alfredo Di Napoli requested to merge adinapoli/ghc:adinapoli/message-class-part-2 into master

This MR builds on top of !4932 (closed) and further reaps the benefits of the new MessageClass/Severity split. In particular, we try to construct virtually all the diagnostics with the correct Severity set "at birth". We are not quite there yet, but very, very close. In particular, @rae will be pleased to know the calls to demoteError are gone. We now have a set_severity local function that does pretty much the same thing, ,but it's arguably cleaner and we can get rid of it, with a bit of work.

In particular, I have spent quite a bit of time refactoring mkHoleError and its ancillary logic, and I think this is paying off.

The only obstacle now remaining is maybeReportError and all the related logic, but hopefully the same reasoning can apply. Having said that, I think that here perfect is the enemy of good, so I suggest we tackle the maybeReportError in another MR, and start enjoying the benefits of the MessageClass split.

NOTA BENE for Richard: The new commits from !4932 (closed) are only from 75506b493425999b3e983aef19ff5f918aec21ee upwards.

Edited by Alfredo Di Napoli

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