[5.7] Remove (?X) and (?u) for now#510

Azoy merged 2 commits intoswiftlang:swift/release/5.7swiftlang/swift-experimental-string-processing:swift/release/5.7from

Azoy:remove-x-u-57Azoy/swift-experimental-string-processing:remove-x-u-57Copy head branch name to clipboard