Use .superType instead of .underlying

Underlying breaks kinds (the underlying of an AppliedType is a HKTypeLambda).
The edited test case was making use of that bug to implement an
impossible match type. Indeed, that test is trying to distinguish

f[Dummy] (for some f)

and

Int

But there is no way to tell that these two types are disjoint.  In fact
they are not, since `type F[X] = Any` matches the `f[Dummy]` pattern and
overlaps with `Int`.

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2508,11 +2508,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         !(tp2 <:< tp1)
         && (provablyDisjoint(tp1, tp2.tp2) || provablyDisjoint(tp1, tp2.tp1))
       case (tp1: TypeProxy, tp2: TypeProxy) =>
-        provablyDisjoint(tp1.underlying, tp2) || provablyDisjoint(tp1, tp2.underlying)
+        provablyDisjoint(tp1.superType, tp2) || provablyDisjoint(tp1, tp2.superType)
       case (tp1: TypeProxy, _) =>
-        provablyDisjoint(tp1.underlying, tp2)
+        provablyDisjoint(tp1.superType, tp2)
       case (_, tp2: TypeProxy) =>
-        provablyDisjoint(tp1, tp2.underlying)
+        provablyDisjoint(tp1, tp2.superType)
       case _ =>
         false
     }

tests/run-macros/tasty-simplified.check

@@ -1,4 +1,4 @@
-Functor[[A >: scala.Nothing <: scala.Any] => scala.collection.immutable.List[A]]
+Functor[Const[scala.collection.immutable.List[Dummy]]]
 Functor[Const[scala.Int]]
-Functor[Id]
-Functor[[A >: scala.Nothing <: scala.Any] => scala.Option[A]]
+Functor[Const[Dummy]]
+Functor[Const[scala.Option[Dummy]]]