[5.7] Improve Unicode/UTS18 and semantic level support (#268)#405

hamishknight wants to merge 1 commit intoswiftlang:swift/release/5.7swiftlang/swift-experimental-string-processing:swift/release/5.7from

hamishknight:268-5.7hamishknight/swift-experimental-string-processing:268-5.7Copy head branch name to clipboard