Printing hack for case _ =>

This is a hack to tell `case _ =>` and `case Any =>` apart while printing match
type error messages. At this point in the compilation pipeline both `pat` types
=:= Any, however because of the slight difference in representation we are able
to restore the original syntax. Here is what `pat.toString` gives in each case:

    // case _ =>
    (TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Any))

    // case Any =>
    (TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Any))

The second case doesn't `eq defn.AnyType` because it starts from <root>, and
thus doesn't match the pattern added in this commit.

Author: OlivierBlanvillain

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

@@ -84,6 +84,7 @@ object MatchTypeTrace:
 
   private def caseText(tp: Type)(using Context): String = tp match
     case tp: HKTypeLambda => caseText(tp.resultType)
+    case defn.MatchCase(any, body) if any eq defn.AnyType => i"case _ => $body"
     case defn.MatchCase(pat, body) => i"case $pat => $body"
     case _ => i"case $tp"