Rollup merge of #57234 - Centril:const-stabilizations-2, r=oli-obk

Const-stabilize `const_int_ops` + `const_ip`

r? @oli-obk

I've added T-lang since this affects intrinsics and the operational semantics of Rust's `const fn` fragment.
This PR depends on #57105 but the FCP intent does not.

## Stable APIs proposed for constification

+ `const_int_ops`:
    + `count_ones`
    + `count_zeros`
    + `leading_zeros`
    + `trailing_zeros`
    + `swap_bytes`
    + `from_be`
    + `from_le`
    + `to_be`
    + `to_le`
+ `const_ip`
    + `Ipv4Addr::new`

## Unstable APIs constified

+ `const_int_conversion`:
    + `reverse_bits`

Author: Centril

src/libcore/intrinsics.rs

@@ -1348,7 +1348,7 @@ extern "rust-intrinsic" {
     /// use std::intrinsics::ctlz;
     ///
     /// let x = 0b0001_1100_u8;
-    /// let num_leading = unsafe { ctlz(x) };
+    /// let num_leading = ctlz(x);
     /// assert_eq!(num_leading, 3);
     /// ```
     ///
@@ -1360,7 +1360,7 @@ extern "rust-intrinsic" {
     /// use std::intrinsics::ctlz;
     ///
     /// let x = 0u16;
-    /// let num_leading = unsafe { ctlz(x) };
+    /// let num_leading = ctlz(x);
     /// assert_eq!(num_leading, 16);
     /// ```
     pub fn ctlz<T>(x: T) -> T;
@@ -1391,7 +1391,7 @@ extern "rust-intrinsic" {
     /// use std::intrinsics::cttz;
     ///
     /// let x = 0b0011_1000_u8;
-    /// let num_trailing = unsafe { cttz(x) };
+    /// let num_trailing = cttz(x);
     /// assert_eq!(num_trailing, 3);
     /// ```
     ///
@@ -1403,7 +1403,7 @@ extern "rust-intrinsic" {
     /// use std::intrinsics::cttz;
     ///
     /// let x = 0u16;
-    /// let num_trailing = unsafe { cttz(x) };
+    /// let num_trailing = cttz(x);
     /// assert_eq!(num_trailing, 16);
     /// ```
     pub fn cttz<T>(x: T) -> T;

src/libcore/lib.rs

@@ -71,7 +71,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
 #![feature(const_fn)]
-#![feature(const_int_ops)]
+#![cfg_attr(stage0, feature(const_int_ops))]
 #![feature(const_fn_union)]
 #![feature(custom_attribute)]
 #![feature(doc_cfg)]
@@ -114,9 +114,7 @@
 #![feature(const_slice_len)]
 #![feature(const_str_as_bytes)]
 #![feature(const_str_len)]
-#![feature(const_int_rotate)]
-#![feature(const_int_wrapping)]
-#![feature(const_int_sign)]
+#![cfg_attr(stage0, feature(const_int_rotate))]
 #![feature(const_int_conversion)]
 #![feature(const_transmute)]
 #![feature(reverse_bits)]