Fix #2948: Use symbol's info when mapping inherited denotations

The test case shows that it is inadmissible to combine the infos of inherited denotations
into a new denotation. The problem here is that a type parameter CC was instantiated in
an inherited denotation to Traversable, yet the parameter was afterwards instantiated to
ListBuffer. This shows that infos from inheroted denotations are useless; instead we have
to go back to the inherited symbol's infos and map them with an asSeenFrom.

This fix also shows that one of the reasons for abandoning #2947 was wrong. We cannot form
denotations from parent denotations in any case, so instantiating early should be fine with
the change in this commit.

Author: odersky

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

@@ -1005,7 +1005,7 @@ object Denotations {
         case _ => if (symbol.exists) symbol.owner else NoSymbol
       }
       if (!owner.membersNeedAsSeenFrom(pre)) this
-      else derivedSingleDenotation(symbol, info.asSeenFrom(pre, owner))
+      else derivedSingleDenotation(symbol, symbol.info.asSeenFrom(pre, owner))
     }
 
     private def overlaps(fs: FlagSet)(implicit ctx: Context): Boolean = this match {