Piecemeal adoption of Swift 6 improvements in Swift 5.x

[Douglas_Gregor]

@Douglas_Gregor

Thank you everyone for the great feedback. I turned this into a proper proposal over at Initial version by DougGregor · Pull Request #1660 · apple/swift-evolution · GitHub .

And for those interested in the implementation details, I have a pull request up at [SE-0362] Piecemeal adoption of upcoming language improvements by DougGregor · Pull Request #59055 · apple/swift · GitHub for the compiler pieces.

Doug