Fix scala#3614: Do not allow the SyntaxFormatter to recolor code

Author: nicolasstucki

compiler/src/dotty/tools/dotc/printing/Highlighting.scala

@@ -53,6 +53,7 @@ object Highlighting {
   }
 
   case class NoColor(text: String) extends Highlight(Console.RESET)
+  case class AlreadyColored(text: String) extends Highlight(Console.RESET)
 
   case class Red(text: String) extends Highlight(Console.RED)
   case class Blue(text: String) extends Highlight(Console.BLUE)

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -719,7 +719,7 @@ object messages {
       .map(_.paramName.unexpandedName.show)
       .mkString("[", ", ", "]")
 
-    private val actualArgString = actual.map(_.show).mkString("[", ", ", "]")
+    private val actualArgString = AlreadyColored(actual.map(_.show).mkString("[", ", ", "]"))
 
     private val prettyName =
       try