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, 1 insertions, 1 deletions

diff --git a/compiler/GHC/Tc/Gen/Bind.hs b/compiler/GHC/Tc/Gen/Bind.hs
index 27572b2a65..228c3d3644 100644
--- a/compiler/GHC/Tc/Gen/Bind.hs
+++ b/compiler/GHC/Tc/Gen/Bind.hs
@@ -1015,7 +1015,7 @@ warnMissingSignatures :: WarningFlag -> SDoc -> Id -> TcM ()
 warnMissingSignatures flag msg id
   = do  { env0 <- tcInitTidyEnv
         ; let (env1, tidy_ty) = tidyOpenType env0 (idType id)
-        ; addWarnTcM (Reason flag) (env1, mk_msg tidy_ty) }
+        ; addDiagnosticTcM (WarningWithFlag flag) (env1, mk_msg tidy_ty) }
   where
     mk_msg ty = sep [ msg, nest 2 $ pprPrefixName (idName id) <+> dcolon <+> ppr ty ]