Fix Erasure's transformDenot for SingleDenotations

They wold transform the pre-existing type of the denotation instead of the
symbol's type. This is clearly the wrong behavior for erasure. It was not
detected before because we changed all types from name-based to symbolic
during erasure, which meant we computed the denotation after erasure by a
loadDenot. So we never exercised the `d.current` code on a derived denotation.

Author: odersky

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

@@ -86,7 +86,7 @@ class Erasure extends Phase with DenotTransformer {
         }
       }
     case ref =>
-      ref.derivedSingleDenotation(ref.symbol, transformInfo(ref.symbol, ref.info))
+      ref.derivedSingleDenotation(ref.symbol, transformInfo(ref.symbol, ref.symbol.info))
   }
 
   val eraser = new Erasure.Typer