[SE-0306] Require cross-actor access to lets to be asynchronous (#1339)
[SE-0306] Require cross-actor access to lets to be asynchronous (#1339)

Per Core Team decision, require that cross-access access to `let`
properties be asynchronous. This brings the proposal into line with
what was accepted.
abertelrud/swift-evolution,apple/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution