Implement mkString on SignedNames

It should not happen that we need to turn signed names into strings.
But it did because of the bug that was fixed in the previous commit.
Anyway, toString is the wrong place to get picky about what should
or should not have happened.

Author: odersky

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

@@ -305,7 +305,7 @@ object NameKinds {
       case _ => None
     }
 
-    def mkString(underlying: TermName, info: ThisInfo): String = unsupported("mkString")
+    def mkString(underlying: TermName, info: ThisInfo): String = s"$underlying[with sig ${info.sig}]"
     def infoString: String = "Signed"
   }