Let ErrorMessageId extend java.lang.Enum

This is necessary so that enum constants come with the class not the companion object
which was in turn necessary to let the bootstrapped build pass.

Two questions:
1) It seems the bootstrapped build got confused. It seemed it compiled against the
previous ErrorMesssageID.java and ran against the ErrorMessageID.scala.
2) In any case, it would be good if enum constants were always generated at the
same place, no matter whether we extend java.lang.Enum or not.

Author: odersky

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

@@ -1,7 +1,7 @@
 package dotty.tools.dotc.reporting.diagnostic
 
 /** Unique IDs identifying the messages */
-enum ErrorMessageID {
+enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
 
   // IMPORTANT: Add new IDs only at the end and never remove IDs
   case