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

4 files changed, 20 insertions, 16 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/SplicesUsed.stderr b/testsuite/tests/partial-sigs/should_compile/SplicesUsed.stderr
index 7a0ad230f4..285060c0c9 100644
--- a/testsuite/tests/partial-sigs/should_compile/SplicesUsed.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/SplicesUsed.stderr
@@ -34,7 +34,7 @@ SplicesUsed.hs:10:16: warning: [-Wpartial-type-signatures (in -Wdefault)]
       In the type signature: charA :: a -> (_)
 
 SplicesUsed.hs:13:13: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘[a]’
+    • Found type wildcard ‘_’ standing for ‘a -> Bool’
       Where: ‘a’ is a rigid type variable bound by
                the inferred type of filter' :: (a -> Bool) -> [a] -> [a]
                at SplicesUsed.hs:14:1-16
@@ -50,7 +50,7 @@ SplicesUsed.hs:13:13: warning: [-Wpartial-type-signatures (in -Wdefault)]
       In the type signature: filter' :: (_ -> _ -> _)
 
 SplicesUsed.hs:13:13: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘a -> Bool’
+    • Found type wildcard ‘_’ standing for ‘[a]’
       Where: ‘a’ is a rigid type variable bound by
                the inferred type of filter' :: (a -> Bool) -> [a] -> [a]
                at SplicesUsed.hs:14:1-16
@@ -58,26 +58,27 @@ SplicesUsed.hs:13:13: warning: [-Wpartial-type-signatures (in -Wdefault)]
       In the type signature: filter' :: (_ -> _ -> _)
 
 SplicesUsed.hs:16:2: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘a -> a -> Bool’
+    • Found extra-constraints wildcard standing for ‘Eq a’
       Where: ‘a’ is a rigid type variable bound by
                the inferred type of foo :: Eq a => a -> a -> Bool
                at SplicesUsed.hs:16:2-11
     • In the type signature: foo :: _ => _
 
 SplicesUsed.hs:16:2: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found extra-constraints wildcard standing for ‘Eq a’
+    • Found type wildcard ‘_’ standing for ‘a -> a -> Bool’
       Where: ‘a’ is a rigid type variable bound by
                the inferred type of foo :: Eq a => a -> a -> Bool
                at SplicesUsed.hs:16:2-11
     • In the type signature: foo :: _ => _
 
 SplicesUsed.hs:18:2: warning: [-Wpartial-type-signatures (in -Wdefault)]
+    • Found type wildcard ‘_a’ standing for ‘Bool’
+    • In the type signature: bar :: _a -> _b -> (_a, _b)
+
+SplicesUsed.hs:18:2: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_b’ standing for ‘w’
       Where: ‘w’ is a rigid type variable bound by
                the inferred type of bar :: Bool -> w -> (Bool, w)
                at SplicesUsed.hs:18:2-11
     • In the type signature: bar :: _a -> _b -> (_a, _b)
 
-SplicesUsed.hs:18:2: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_a’ standing for ‘Bool’
-    • In the type signature: bar :: _a -> _b -> (_a, _b)
diff --git a/testsuite/tests/partial-sigs/should_compile/T14643.stderr b/testsuite/tests/partial-sigs/should_compile/T14643.stderr
index e2dd144bd3..9c56cb629f 100644
--- a/testsuite/tests/partial-sigs/should_compile/T14643.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T14643.stderr
@@ -1,8 +1,9 @@
 
 T14643.hs:5:18: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found extra-constraints wildcard standing for ‘()’
-    • In the type signature: ag :: (Num a, _) => a -> a
+    • In the type signature: af :: (Num a, _) => a -> a
 
 T14643.hs:5:18: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found extra-constraints wildcard standing for ‘()’
-    • In the type signature: af :: (Num a, _) => a -> a
+    • In the type signature: ag :: (Num a, _) => a -> a
+
diff --git a/testsuite/tests/partial-sigs/should_compile/T16728a.stderr b/testsuite/tests/partial-sigs/should_compile/T16728a.stderr
index a23c189c4b..2c7e597623 100644
--- a/testsuite/tests/partial-sigs/should_compile/T16728a.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T16728a.stderr
@@ -7,7 +7,7 @@ T16728a.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
                  h :: a -> w
                at T16728a.hs:(5,1)-(7,9)
     • In the type ‘a -> _’
-      In the type signature: h :: forall a. a -> _
+      In the type signature: g :: forall a. a -> _
 
 T16728a.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘w’
@@ -17,4 +17,5 @@ T16728a.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
                  h :: a -> w
                at T16728a.hs:(5,1)-(7,9)
     • In the type ‘a -> _’
-      In the type signature: g :: forall a. a -> _
+      In the type signature: h :: forall a. a -> _
+
diff --git a/testsuite/tests/partial-sigs/should_compile/T16728b.stderr b/testsuite/tests/partial-sigs/should_compile/T16728b.stderr
index 9948e78198..84d10c1f45 100644
--- a/testsuite/tests/partial-sigs/should_compile/T16728b.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T16728b.stderr
@@ -1,13 +1,14 @@
 
 T16728b.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Bool’
-    • In the type ‘a -> _’
-      In the type signature: h :: forall a. a -> _
-
-T16728b.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘a’
       Where: ‘a’ is a rigid type variable bound by
                the inferred type of g :: a -> a
                at T16728b.hs:4:14
     • In the type ‘a -> _’
       In the type signature: g :: forall a. a -> _
+
+T16728b.hs:4:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
+    • Found type wildcard ‘_’ standing for ‘Bool’
+    • In the type ‘a -> _’
+      In the type signature: h :: forall a. a -> _
+