Allow subtyping conversion to add `@Sendable` in preconcurrency code.
Allow subtyping conversion to add `@Sendable` in preconcurrency code.

Fixes rdar://99518344.

roambotics/swift,roambotics/swift,apple/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,roambotics/swift