The current incarncation of DuplicateFinder does work that
AstValueFactory already does. All that remains is that
DuplicateFinder wraps a container.

Adding const-ness changes were necessary to have IsDuplicateSymbol
be const.

BUG=v8:6092

Change-Id: I8081cfeef363717405d5b6325e290fe7725390dc
Reviewed-on: https://chromium-review.googlesource.com/456317
Commit-Queue: Marja Hölttä <marja@chromium.org>
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43872}