Use the strict concurrency level to make decisions.
`isConcurrencyChecked()` was being used as a proxy for
`-warn-concurrency` that didn't account for Swift 6. Replace
checks against it within the current module with checks against the
strict concurrency level, which subsumes the Swift 6 check and can
account for the difference between "limited" and "on".
`isConcurrencyChecked()` is used now used exclusively to mean "treat a
missing Sendable conformance as an explicitly-non-Sendable type".
