Correct owner chains when mixing byname implicits and inlining

[milessabin]

When constructing the dictionary for a byname implicit resolution we have to ensure that the owners of any definitions which have been hoisted into the dictionary are correct. Prior to this PR this was only
done fully for implicit expansions which didn't involve inlining. We fix that here by ensuring that all definitions under a given dictionary entry are owned directly or indirectly by that dictionary entry.

Fixes #5766.

[allanrenucci]

Can this be simplified further?

def changeNonLocalOwners(newOwner: Symbol)(implicit ctx: Context): Tree = {
  val localOwnerAcc = new TreeAccumulator[List[Symbol]] {
    def apply(ss: List[Symbol], tree: Tree)(implicit ctx: Context) = tree match {
      case tree: DefTree =>
        if (tree.symbol.exists) tree.symbol :: ss
        else ss
      case _ =>
        foldOver(ss, tree)
      }
    }
  }
  val nonLocalOwners = localOwnerAcc(Nil, tree).map(_.owner).distinct
  val newOwners = nonLocalOwners.map(_ => newOwner)
  new TreeTypeMap(oldOwners = nonLocalOwners, newOwners = newOwners).apply(tree)
}

[odersky]

I think the idea of using an accumulator is good, but why the detour via lists? Let's use an accumulator over an immutable set instead.

[allanrenucci]

Updated:

def changeNonLocalOwners(newOwner: Symbol)(implicit ctx: Context): Tree = {
  val ownerAcc = new TreeAccumulator[Set[Symbol]] {
    def apply(ss: Set[Symbol], tree: Tree)(implicit ctx: Context) = tree match {
      case tree: DefTree =>
        if (tree.symbol.exists) ss + tree.symbol.owner
        else ss
      case _ =>
        foldOver(ss, tree)
      }
    }
  }
  val owners = ownerAcc(Set.empty, tree)
  val newOwners = List.fill(owners.size)(newOwner)
  new TreeTypeMap(oldOwners = owners.toList, newOwners = newOwners).apply(tree)
}

[odersky]

Otherwise LGTM.

[odersky]

I think the idea of using an accumulator is good, but why the detour via lists? Let's use an accumulator over an immutable set instead.

Incorporated review feedback. I'll merge when green.