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/SysTools/Process.hs b/compiler/GHC/SysTools/Process.hs
index d98710febd..c9cd24cf89 100644
--- a/compiler/GHC/SysTools/Process.hs
+++ b/compiler/GHC/SysTools/Process.hs
@@ -310,7 +310,7 @@ builderMainLoop logger dflags filter_fn pgm real_args mb_cwd mb_env = do
           logInfo logger dflags $ withPprStyle defaultUserStyle msg
           log_loop chan t
         BuildError loc msg -> do
-          putLogMsg logger dflags NoReason SevError (mkSrcSpan loc loc)
+          putLogMsg logger dflags (mkMCDiagnostic ErrorWithoutFlag) (mkSrcSpan loc loc)
               $ withPprStyle defaultUserStyle msg
           log_loop chan t
         EOF ->