Rollup merge of #92879 - compiler-errors:into_iter_unsound, r=dtolnay

Add Sync bound to allocator parameter in vec::IntoIter

The `A: Sync` bound was forgotten in 8725e4c#diff-b78c3ab6d37f4ede32195707528f8a76c49d4557cc9d3a7a09417b5157729b9fR3132

Similar `unsafe impl Sync` in that commit _do_ include the `A: Sync` bound (and around the alloc lib), so I think this was just an honest mistake.

Here's an example of the unsoundness:  https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=16cbfeff7c934ae72ab632c1476fdd8b

`@steffahn` found this, I'm just putting up the fix cause nobody else did :^)

Fixes #92633

library/alloc/src/vec/into_iter.rs

@@ -125,7 +125,7 @@ impl<T, A: Allocator> AsRef<[T]> for IntoIter<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Send, A: Allocator + Send> Send for IntoIter<T, A> {}
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Sync, A: Allocator> Sync for IntoIter<T, A> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for IntoIter<T, A> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> Iterator for IntoIter<T, A> {