Add Sender diagnostic item for std::sync::mpsc::Sender

Similar to the existing `Receiver` item, it will be used in Clippy to
detect uses of `is_disconnected` that are racy.

library/std/src/sync/mpsc.rs

@@ -330,6 +330,7 @@ pub struct IntoIter<T> {
 /// assert_eq!(3, msg + msg2);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "MpscSender")]
 pub struct Sender<T> {
     inner: mpmc::Sender<T>,
 }