[stdlib] Use unsigned arithmetic in partition

Author: natecook1000

stdlib/public/core/Sort.swift.gyb

@@ -165,8 +165,8 @@ func _partition<C>(
 
   // Sort the first, middle, and last elements, then use the middle value
   // as the pivot for the partition.
-  let half = elements.distance(from: lo, to: hi) / 2
-  let mid = elements.index(lo, offsetBy: half)
+  let half = numericCast(elements.distance(from: lo, to: hi)) as UInt / 2
+  let mid = elements.index(lo, offsetBy: numericCast(half))
   _sort3(&elements, lo, mid, hi
     ${", by: areInIncreasingOrder" if p else ""})
   let pivot = elements[mid]