Make elim opaque transform declare that it changes base types

Author: odersky

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

@@ -42,6 +42,7 @@ class FirstTransform extends MiniPhase with SymTransformer { thisPhase =>
   import ast.tpd._
 
   override def phaseName: String = FirstTransform.name
+  override def changesParents: Boolean = true // the phase drops Any
 
   /** Two transforms:
    *   1. eliminate self symbol in ClassInfo