add #[track_caller] to slice split_at_mut fn

Author: Rustin170506

library/core/src/slice/mod.rs

@@ -1530,6 +1530,7 @@ impl<T> [T] {
     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[track_caller]
     #[inline]
     pub fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
         assert!(mid <= self.len());