Add `MessageClass`, rework `Severity` and add `DiagnosticReason`.wip/adinapoli-message-class-new-design

Other than that:

* Fix T16167,json,json2,T7478,T10637 tests to reflect the introduction of
  the `MessageClass` type

* Remove `makeIntoWarning`

* Remove `warningsToMessages`

* Refactor GHC.Tc.Errors

  1. Refactors GHC.Tc.Errors so that we use `DiagnosticReason` for "choices"
     (defer types errors, holes, etc);

  2. We get rid of `reportWarning` and `reportError` in favour of a general
     `reportDiagnostic`.

* Introduce `DiagnosticReason`, `Severity` is an enum: This big commit makes
  `Severity` a simple enumeration, and introduces the concept of `DiagnosticReason`,
  which classifies the /reason/ why we are emitting a particular diagnostic.
  It also adds a monomorphic `DiagnosticMessage` type which is used for
  generic messages.

* The `Severity` is computed (for now) from the reason, statically.
  Later improvement will add a `diagReasonSeverity` function to compute
  the `Severity` taking `DynFlags` into account.

* Rename `logWarnings` into `logDiagnostics`

* Add note and expand description of the `mkHoleError` function

1 files changed, 0 insertions, 23 deletions

diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index b6d20ada3a..393927e1b2 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -2,7 +2,6 @@ module GHC.Driver.Flags
    ( DumpFlag(..)
    , GeneralFlag(..)
    , WarningFlag(..)
-   , WarnReason (..)
    , Language(..)
    , optimisationFlags
    )
@@ -11,7 +10,6 @@ where
 import GHC.Prelude
 import GHC.Utils.Outputable
 import GHC.Data.EnumSet as EnumSet
-import GHC.Utils.Json
 
 -- | Debugging flags
 data DumpFlag
@@ -514,27 +512,6 @@ data WarningFlag =
    | Opt_WarnMissingKindSignatures        -- Since 9.2
    deriving (Eq, Show, Enum)
 
--- | Used when outputting warnings: if a reason is given, it is
--- displayed. If a warning isn't controlled by a flag, this is made
--- explicit at the point of use.
-data WarnReason
-  = NoReason
-  -- | Warning was enabled with the flag
-  | Reason !WarningFlag
-  -- | Warning was made an error because of -Werror or -Werror=WarningFlag
-  | ErrReason !(Maybe WarningFlag)
-  deriving Show
-
-instance Outputable WarnReason where
-  ppr = text . show
-
-instance ToJson WarnReason where
-  json NoReason = JSNull
-  json (Reason wf) = JSString (show wf)
-  json (ErrReason Nothing) = JSString "Opt_WarnIsError"
-  json (ErrReason (Just wf)) = JSString (show wf)
-
-
 data Language = Haskell98 | Haskell2010 | GHC2021
    deriving (Eq, Enum, Show, Bounded)