This is a follow-up to https://github.com/swiftlang/swift/pull/82085
which made it so async variant doesn't get `@Sendable` inferred because
the proposal specified that inference should happen only on completion
handler parameter type of a synchronous variant of an imported API.
This runs into implementation issues related to thunking in some
cases were async convention expects the type of a completion handler
to match exactly for both variants of the imported API.
Resolves: rdar://154695053
