[5.9] Add diagnostic for wrong inheritance#1585

kimdv wants to merge 1 commit intoswiftlang:release/5.9swiftlang/swift-syntax:release/5.9from

kimdv:kimdv/cherry-pick-add-diagnostic-for-wrong-inheritancekimdv/swift-syntax:kimdv/cherry-pick-add-diagnostic-for-wrong-inheritanceCopy head branch name to clipboard