Rustdoc does not wait for DocFS writes to complete before exiting on Windows

[willcrichton]

As discovered by @ehuss in rust-lang/cargo#10120, a Cargo test is intermittently failing on Windows because a source file generated by Rustdoc is not being written before the process exits. The suspected issue is that Rustdoc spawns asynchronous writes here:

rust/src/librustdoc/docfs.rs

Lines 62 to 68 in f41927f

	rayon::spawn(move || {
	fs::write(&path, contents).unwrap_or_else(|e| {
	sender.send(format!("\"{}\": {}", path.display(), e)).unwrap_or_else(|_| {
	panic!("failed to send error on \"{}\"", path.display())
	})
	});
	});

However, Rustdoc never checks that the writes are completed. We should add a mechanism that prevents the process from exiting before these write tasks are completed.

One possible solution:

• Add a field outstanding_writes: Arc<AtomicUsize> to DocFS.
• Increment outstanding_writes at the start of DocFS::write, and decrement at the end of the spawned Rayon task.
• Add a method DocFS::wait_for_outstanding_writes(&self) that spin-loops until outstanding_writes is zero. (Alternatively: use a condition variable, and have write notify the condvar when outstanding_writes is decremented.)
• Either call wait_for_outstanding_writes explicitly in the end of the Rustdoc process, or add it as a Drop implementation to DocFS.

cc @GuillaumeGomez