Re-add debug-mode overflow check in index(_:,_offsetBy:limit:)

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -221,8 +221,9 @@ extension Unsafe${Mutable}BufferPointer: ${Mutable}Collection, RandomAccessColle
       return nil
     }
 
-    // No need to check here, for the same index misuse reasons.
-    return i &+ n
+    let result = i.addingReportingOverflow(n)
+    _debugPrecondition(!result.overflow)
+    return result.partialValue
   }
 
   @inlinable // unsafe-performance