[DirectoryWatcher] Add assert for unknown event kinds
[DirectoryWatcher] Add assert for unknown event kinds

https://reviews.llvm.org/D58418

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang