Mark the panic macros as diagnostic items.

library/core/src/macros/mod.rs

@@ -2,6 +2,7 @@
 #[macro_export]
 #[allow_internal_unstable(core_panic, const_caller_location)]
 #[stable(feature = "core", since = "1.6.0")]
+#[rustc_diagnostic_item = "core_panic_macro"]
 macro_rules! panic {
     () => (
         $crate::panic!("explicit panic")

library/std/src/macros.rs

@@ -8,6 +8,7 @@
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow_internal_unstable(libstd_sys_internals)]
+#[rustc_diagnostic_item = "std_panic_macro"]
 macro_rules! panic {
     () => ({ $crate::panic!("explicit panic") });
     ($msg:expr $(,)?) => ({ $crate::rt::begin_panic($msg) });