[stdlib] a minor optimization

We have already done the hard work of unwrapping the optionals
that represent the first and last pointer, and calling `self.count` does it all over again. Use the distance between the unwrapped pointers instead.

Author: glessard

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -161,7 +161,7 @@ extension Unsafe${Mutable}RawBufferPointer: Sequence {
     }
     let destinationAddress = destination.baseAddress._unsafelyUnwrappedUnchecked
     let d = UnsafeMutableRawPointer(destinationAddress)
-    let n = Swift.min(destination.count, self.count)
+    let n = Swift.min(destination.count, s.distance(to: e))
     d.copyMemory(from: s, byteCount: n)
     return (Iterator(_position: s.advanced(by: n), _end: e), n)
   }