DirectoryWatcher: close a possible window of race on Windows

The initial scan occurring before the watcher is ready allows a race
condition where a change occurs before the initial scan completes.
Ensure that we wait for the watcher to begin executing the initial scan.

Addresses some feedback from Adrian McCarthy in post-commit review.

clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp

@@ -133,6 +133,9 @@ DirectoryWatcherWindows::~DirectoryWatcherWindows() {
 }
 
 void DirectoryWatcherWindows::InitialScan() {
+  std::unique_lock<std::mutex> lock(Mutex);
+  Ready.wait(lock, [this] { return this->WatcherActive; });
+
   Callback(getAsFileEvents(scanDirectory(Path.data())), /*IsInitial=*/true);
 }