Add Ipv4Addr and Ipv6Addr diagnostic items

They will be used in Clippy to detect runtime parsing of known-valid
IP addresses.

Author: samueltardieu

core/src/net/ip_addr.rs

@@ -68,6 +68,7 @@ pub enum IpAddr {
 /// assert!("0000000.0.0.0".parse::<Ipv4Addr>().is_err()); // first octet is a zero in octal
 /// assert!("0xcb.0x0.0x71.0x00".parse::<Ipv4Addr>().is_err()); // all octets are in hex
 /// ```
+#[rustc_diagnostic_item = "Ipv4Addr"]
 #[derive(Copy, Clone, PartialEq, Eq)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Ipv4Addr {
@@ -160,6 +161,7 @@ impl Hash for Ipv4Addr {
 /// assert_eq!("::1".parse(), Ok(localhost));
 /// assert_eq!(localhost.is_loopback(), true);
 /// ```
+#[rustc_diagnostic_item = "Ipv6Addr"]
 #[derive(Copy, Clone, PartialEq, Eq)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Ipv6Addr {