Dedupe location names against script-root

[robertbastian]

Fixes #5901

I'm not introducing script fallback (because time zone names are not meant to fall back by script), and I'm not introducing und-Xxxx locales, as those would increase data size (due to the other fields in the struct). Instead, I store a dedupe_locale in each data struct, which contains the locale that was deduplicated against. So for sr, this would be ru, because sr -> und-Cyrl -> ru, and for ru it'd be ru as well, and ru is complete. This way, we also directly know which dedupe locale to load; if we didn't store it we'd have to load a fallbacker and step through it until we hit und (and then use the previous value).

Depends on #5967

[dpulls]

🎉 All dependencies have been resolved !

[dpulls]

🎉 All dependencies have been resolved !

[sffc]

Fixes #5901

Do we not do similar deduplication in the other time zone keys as well?

[sffc]

So the problem with dedupe_locale is that the fallback engine has no knowledge that it needs to keep both payloads around. If you pass LocaleFamily("sr") into datagen, datagen is free to remove ru, but if we redefine the key to script fallback, it will know that it needs to retain und-Cyrl. (well, unless no-ancestor mode is used.)

[sffc]

Hmm. An approach that would work with our framework would be to have two keys:

1. location_base
2. location_override

Every locale has an entry in each key, but location_base has one unique payload per script, and location_override contains smaller payloads, which are empty if the data exactly equals the base.