Relax bounds of rc::Weak::as_ptr and friends

....to allow use with unsized T. This is a follow-up to rust-lang#73845,
which did the impl work required to be able to relax these bounds.

Author: CAD97

src/liballoc/rc.rs

@@ -1675,7 +1675,14 @@ impl<T> Weak<T> {
     pub fn new() -> Weak<T> {
         Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
     }
+}
 
+pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
+    let address = ptr.as_ptr() as *mut () as usize;
+    address == usize::MAX
+}
+
+impl<T: ?Sized> Weak<T> {
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
     /// The pointer is valid only if there are some strong references. The pointer may be dangling,
@@ -1808,14 +1815,7 @@ impl<T> Weak<T> {
             }
         }
     }
-}
 
-pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
-    let address = ptr.as_ptr() as *mut () as usize;
-    address == usize::MAX
-}
-
-impl<T: ?Sized> Weak<T> {
     /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
     /// dropping of the inner value if successful.
     ///