Mark location doctest as standalone since file information will not w…

…ork in merged doctest file

core/src/panic/location.rs

@@ -44,7 +44,7 @@ impl<'a> Location<'a> {
     ///
     /// # Examples
     ///
-    /// ```
+    /// ```standalone
     /// use std::panic::Location;
     ///
     /// /// Returns the [`Location`] at which it is called.