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Use `LikelySubtagsForLanguageV1` for fallback (#5338)

`LikelySubtagsForLanguageV1` is almost identical to
`LocaleFallbackLikelySubtagsV1`, the differences being:
* `LikelySubtagsForLanguageV1` contains a `und` field, which is not
needed for fallback
* `LocaleFallbackLikelySubtagsV1` uses `ZeroMap2d`s, whereas
`LikelySubtagsForLanguageV1` uses `ZeroMap`s with tuple keys. Fallback
always looks up by both keys at the same time, so the `ZeroMap2d`
overhead is unnecessary.
* `LocaleFallbackLikelySubtagsV1` omits Language->Script entries where
the script is `Latn`. While this might save a few bytes, I'm not sure
that it's correct, as fallback for unknown languages will assume that
they are Latin and strip the `Latn` subtag during normalisation.

The `ZeroMap2d` overhead seems to cancel out the `und` field and `Latn`
optimisation, and `LikelySubtagsForLanguageV1` is only two bytes bigger
than `LocaleFallbackLikelySubtagsV1` (2230B vs 2228B).

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