Use unreachable_unchecked in const_panic_fmt.

library/core/src/panicking.rs

@@ -100,7 +100,10 @@ pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if let Some(msg) = fmt.as_str() {
         panic_str(msg);
     } else {
-        panic_str("???");
+        // SAFETY: This is only evaluated at compile time, which handles this
+        // fine (in case it turns out this branch turns out to be reachable
+        // somehow).
+        unsafe { crate::hint::unreachable_unchecked() };
     }
 }