library: Stabilize const_intrinsic_forget

This is an implicit requirement of stabilizing `const_ptr_write`.

Const-stabilizes the internal `core::intrinsics`:
- `forget`

core/src/intrinsics.rs

@@ -1084,7 +1084,7 @@ extern "rust-intrinsic" {
     /// it does not require an `unsafe` block.
     /// Therefore, implementations must not require the user to uphold
     /// any safety invariants.
-    #[rustc_const_unstable(feature = "const_intrinsic_forget", issue = "none")]
+    #[rustc_const_stable(feature = "const_intrinsic_forget", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
     pub fn forget<T: ?Sized>(_: T);

core/src/lib.rs

@@ -124,7 +124,6 @@
 #![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_index_range_slice_index)]
-#![feature(const_intrinsic_forget)]
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]