Add const_cell_into_inner to OnceCell

`Cell` and `RefCell` have their `into_inner` methods const unstable.
`OnceCell` has the same logic, so add it under the same gate.

Tracking issue: rust-lang#78729

Author: tgross35

library/core/src/cell/once.rs

@@ -309,7 +309,8 @@ impl<T> OnceCell<T> {
     /// ```
     #[inline]
     #[stable(feature = "once_cell", since = "1.70.0")]
-    pub fn into_inner(self) -> Option<T> {
+    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
+    pub const fn into_inner(self) -> Option<T> {
         // Because `into_inner` takes `self` by value, the compiler statically verifies
         // that it is not currently borrowed. So it is safe to move out `Option<T>`.
         self.inner.into_inner()