Re-Revert "DirectoryWatcher: add an implementation for Windows"

This reverts commit 76f1baa7.

Also reverts 2 follow-ups:

1. Revert "DirectoryWatcher: also wait for the notifier thread"
   This reverts commit 527a1821.

2. Revert "DirectoryWatcher: close a possible window of race on Windows"
   This reverts commit a6948da8.

Makes tests hang, see comments on https://reviews.llvm.org/D88666