Rollup merge of rust-lang#32745 - Amanieu:arc_fix, r=alexcrichton

Fix infinite loop in Arc::downgrade

src/liballoc/arc.rs

@@ -259,6 +259,7 @@ impl<T: ?Sized> Arc<T> {
         loop {
             // check if the weak counter is currently "locked"; if so, spin.
             if cur == usize::MAX {
+                cur = this.inner().weak.load(Relaxed);
                 continue;
             }