[DirectoryWatcher] Fix misuse of FSEvents API and data race

I observed two bugs in the DirectoryWatcher on macOS

1. We were calling FSEventStreamStop and FSEventStreamInvalidate before
we called FSEventStreamStart and FSEventStreamSetDispatchQueue, if the
DirectoryWatcher was destroyed before the initial async work was done.
This violates the requirements of the FSEvents API.

2. Calls to Receiver could race between the initial work and the
invalidation during destruction.

The second issue is easier to see when using TSan.

Differential Revision: https://reviews.llvm.org/D74371

rdar://59215667