simplify the sendable / initializer relationship
simplify the sendable / initializer relationship

I decided later in the review process that the initially-proposed rules were too complex for what was gained. This commit changes the rules to match my change-of-heart as posted here:

https://forums.swift.org/t/se-0327-second-review-on-actors-and-initialization/54093/8

apple/swift-evolution,abertelrud/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution