Fix bug with assert!() calling the wrong edition of panic!().

The span of `panic!` produced by the `assert` macro did not carry the
right edition. This changes `assert` to call the right version.

Author: m-ou-se

core/src/macros/mod.rs

@@ -1234,7 +1234,7 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[macro_export]
     #[rustc_diagnostic_item = "assert_macro"]
-    #[allow_internal_unstable(core_panic)]
+    #[allow_internal_unstable(core_panic, edition_panic)]
     macro_rules! assert {
         ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
         ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};