Rollup merge of #95965 - CAD97:const-weak-new, r=workingjubilee

Stabilize const-weak-new

This is a fairly uncontroversial library stabilization, so I'm going ahead and proposing it to ride the trains to stable.

This stabilizes the following APIs, which are defined to be non-allocating constructors.

```rust
// alloc::rc
impl<T> Weak<T> {
    pub const fn new() -> Weak<T>;
}

// alloc::sync
impl<T> Weak<T> {
    pub const fn new() -> Weak<T>;
}
```

Closes #95091

``@rustbot`` modify labels: +needs-fcp

library/alloc/src/rc.rs

@@ -2719,7 +2719,7 @@ impl<T> Weak<T> {
     /// ```
     #[inline]
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
-    #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
+    #[rustc_const_stable(feature = "const_weak_new", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {

library/alloc/src/sync.rs

@@ -2503,7 +2503,7 @@ impl<T> Weak<T> {
     /// ```
     #[inline]
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
-    #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
+    #[rustc_const_stable(feature = "const_weak_new", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {