FindMember could defer call to parentSymbols until needed

[retronym]

As findMember walks up the base class sequence, it records the direct parent symbols of the first encounted refinement class.

        // SLS 5.2 The private modifier can be used with any definition or declaration in a template.
        //         They are not inherited by subclasses [...]
        if (currentBaseClass.isRefinementClass)
          // SLS 3.2.7 A compound type T1 with . . . with Tn {R } represents objects with members as given in
          //           the component types T1, ..., Tn and the refinement {R }
          //
          //           => private members should be included from T1, ... Tn. (scala/bug#7475)
          refinementParents :::= currentBaseClass.parentSymbols

This list is later used to deal with a corner case (whether a private member should be inherited or not)

      def admitPrivate(sym: Symbol): Boolean =
        (selectorClass == owner) || (
             !isPrivateLocal // private[this] only a member from within the selector class. (Optimization only? Does the spec back this up?)
          && (
                  !seenFirstNonRefinementClass
               || refinementParents.contains(owner)
             )
        )

This call to parentSymbols accounts for 1.5% of allocated :: instances. We should refactor this code to defer the until we actually end up dealing with this corner case.

[diesalbla]

@retronym I have been looking at this issue, and I am working on a possible fix.