Populate seen_names hash in cooked_index_shard::do_finalize

Populate seen_names hash in cooked_index_shard::do_finalize

Hannes pointed out that cooked_index_shard::do_finalize never
populates the seen_names hash table.  This patch adds the necessary
store.  This reduces memory use a little for "gdb gdb":

(before) Space used: 28909568 (+0 for this command)
(after)  Space used: 28884992 (+0 for this command)

What this means, btw, is that in gdb there are not many symbols that
are both mentioned in many CUs and that also require name
canonicalization.  It's possible this would differ in other programs.