Rollup merge of #129673 - matthewpipie:arc-weak-debug-trait, r=dtolnay

Add fmt::Debug to sync::Weak<T, A>

Currently, `sync::Weak<T>` implements `Debug`, but `sync::Weak<T, A>` does not. This appears to be an oversight, as `rc::Weak<T, A>` implements `Debug`. (Note: `sync::Weak` is the weak for `Arc`, and `rc::Weak` is the weak for `Rc`.)

This PR adds the Debug trait for `sync::Weak<T, A>`. The issue was initially brought up here: rust-lang/wg-allocators#131