Auto merge of #34539 - arielb1:metadata-hash, r=alexcrichton

Make the metadata lock more robust

Fixes #33778 and friends.

I also needed to add a metadata encoding version to rlibs, as they did not have it before. To keep it backwards-compatible, I added 4 zeroes to the start of the metadata, which are treated as an empty length field by older rustcs.

r? @alexcrichton

src/librustc/dep_graph/thread.rs

@@ -118,8 +118,6 @@ impl DepGraphThreadData {
     /// the buffer is full, this may swap.)
     #[inline]
     pub fn enqueue(&self, message: DepMessage) {
-        debug!("enqueue: {:?} tasks_pushed={}", message, self.tasks_pushed.get());
-
         // Regardless of whether dep graph construction is enabled, we
         // still want to check that we always have a valid task on the
         // stack when a read/write/etc event occurs.