I was just working on a patch and incorrectly assumed that we had no data structure that could support an unordered set of unowned strings (StringData) other than this one: StringData::ComparatorInterface::StringDataUnorderedSet.

It turns out that `StringDataSet` is the same thing, but without the collator interface virtual method overhead. In my context I never needed to support collation, so this would have been the better choice.

To improve visibility and discoverability, I would suggest `UnorderedStringSet` and `UnorderedStringDataSet`.