[Intl] made RegionBundle and LanguageBundle merge fallback data when using a country-specific locale
Merge branch '2.3'

* 2.3:
  [HttpKernel] added a check for private event listeners/subscribers
  [FrameworkBundle] fixed registration of the register listener pass
  [Form] Fixed regression causing invalid "WHERE id IN ()" statements
  [DependencyInjection] fixed a non-detected circular reference in PhpDumper (closes #8425)
  [Form] Fixed regression in BooleanToStringTransformer from ed83752
  [FrameworkBundle] removed obsolete code
  [Process] Close unix pipes before calling `proc_close` to avoid a deadlock
  [Process] Fix process merge in 2.3
  [Intl] made RegionBundle and LanguageBundle merge fallback data when using a country-specific locale