[Intl] made RegionBundle and LanguageBundle merge fallback data when using a country-specific locale
merged branch shieldo/country_fallback_locale (PR #8564)

This PR was merged into the 2.3 branch.

Discussion
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[Intl] made RegionBundle merge fallback data if using a country-specific locale

See #8442 and symfony/Icu#2.

Essentially, country data fetches from the Intl component do not currently work when using a locale with a country specifier, e.g. `fr_FR`.  This change forces a merge on the data against the root language locale, thus making country data available for such locales.

Commits
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52d8676 [Intl] made RegionBundle and LanguageBundle merge fallback data when using a country-specific locale