[Second Review] SE-0410: Atomics

[hborla (Holly Borla)]

@taylorswift

not true, @unchecked Sendable applies to the whole type, so adding a single atomic stored property to an otherwise Sendable class means the whole class needs to opt out of Sendable checking.

Note that with SE-0412 you can instead apply nonisolated(unsafe) to the atomic stored property and still use a checked Sendable conformance.