[DirectoryWatcher] Replace buffer length macro with constexpr
[DirectoryWatcher] Replace buffer length macro with constexpr

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