Fix diagnostics breakage

Someone changed TastyPrinter.printContents to return a string without
actually printing anything. Furthermore this was done without checking
who else uses the function, which means that importan diagnostics broke.
Thank you very much! I spent about an hour figuring out what went wrong.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -78,7 +78,7 @@ class Pickler extends Phase {
       // println(i"rawBytes = \n$rawBytes%\n%") // DEBUG
       if (pickling ne noPrinter) {
         println(i"**** pickled info of $cls")
-        new TastyPrinter(pickler.assembleParts()).printContents()
+        println(new TastyPrinter(pickler.assembleParts()).printContents())
       }
     }
   }