Make delegation methods of std::net::IpAddr unstable const

Make the following methods of `std::net::IpAddr` unstable const under the `const_ip` feature:
- `is_unspecified`
- `is_loopback`
- `is_global`
- `is_multicast`

Also adds a test for these methods in a const context.

Possible because these methods delegate to the inner `Ipv4Addr` or `Ipv6Addr`, which were made const, and the recent stabilization of const control flow.

Part of #76205

library/std/src/lib.rs

@@ -238,6 +238,7 @@
 #![feature(const_cstr_unchecked)]
 #![feature(const_fn_transmute)]
 #![feature(const_fn)]
+#![feature(const_ip)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]
 #![feature(const_ipv4)]