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

2 files changed, 6 insertions, 6 deletions

diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index a028f4e479..a97200c5c3 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -577,10 +577,10 @@ resetLastErrorLocations = do
 
 ghciLogAction :: IORef [(FastString, Int)] -> LogAction -> LogAction
 ghciLogAction lastErrLocations old_log_action
-              dflags flag severity srcSpan msg = do
-    old_log_action dflags flag severity srcSpan msg
-    case severity of
-        SevError -> case srcSpan of
+              dflags msg_class srcSpan msg = do
+    old_log_action dflags msg_class srcSpan msg
+    case msg_class of
+        MCDiagnostic SevError _reason -> case srcSpan of
             RealSrcSpan rsp _ -> modifyIORef lastErrLocations
                 (++ [(srcLocFile (realSrcSpanStart rsp), srcLocLine (realSrcSpanStart rsp))])
             _ -> return ()
@@ -3188,7 +3188,7 @@ showCmd str = do
             , action "bindings"   $ showBindings
             , action "linker"     $ do
                msg <- liftIO $ Loader.showLoaderState (hscInterp hsc_env)
-               putLogMsgM NoReason SevDump noSrcSpan msg
+               putLogMsgM MCDump noSrcSpan msg
             , action "breaks"     $ showBkptTable
             , action "context"    $ showContext
             , action "packages"   $ showUnits
diff --git a/ghc/Main.hs b/ghc/Main.hs
index 9da5469b8a..2873cba4ad 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -23,7 +23,7 @@ import GHC.Driver.CmdLine
 import GHC.Driver.Env
 import GHC.Driver.Errors
 import GHC.Driver.Phases
-import GHC.Driver.Session hiding (WarnReason(..))
+import GHC.Driver.Session
 import GHC.Driver.Ppr
 import GHC.Driver.Pipeline  ( oneShot, compileFile )
 import GHC.Driver.MakeFile  ( doMkDependHS )