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ARROW-12554: [C++] Allow duplicates in `SetLookupOptions::value_set`

For the `index_in` function, we need to map the memo table indices to indices in the value_set
(they are different in there are duplicates).

This fixes the current benchmark failures for `is_in` and `index_in`.

Closes #10174 from pitrou/ARROW-12554-index-in-duplicates

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>