Avoid forcing diagnostic message in ErrorType

I found the problem when trying to debug what went wrong with encoded names
in backend. I added an assert that the toString of a name would not contain
symbolic operator characters. The assert triggered because an error type forced
the message (without printing to Console later).

Author: odersky

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

@@ -3306,7 +3306,7 @@ object Types {
   abstract class FlexType extends UncachedGroundType with ValueType
 
   class ErrorType(_msg: => Message) extends FlexType {
-    val msg = _msg
+    def msg = _msg
   }
 
   object UnspecifiedErrorType extends ErrorType("unspecified error")