Use BorrowFlag instead of explicit isize

The integer type tracking borrow count has a typedef called `BorrowFlag`. This type should be used instead of explicit `isize`.

Author: Danvil

library/core/src/cell.rs

@@ -1756,7 +1756,7 @@ impl<'b> BorrowRefMut<'b> {
         let borrow = self.borrow.get();
         debug_assert!(is_writing(borrow));
         // Prevent the borrow counter from underflowing.
-        assert!(borrow != isize::MIN);
+        assert!(borrow != BorrowFlag::MIN);
         self.borrow.set(borrow - 1);
         BorrowRefMut { borrow: self.borrow }
     }