Zhihao Yuan noted that a move assignment operation was missing from std::adjacent_difference.  Fixed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189036 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/numeric b/include/numeric
index c201a5f..e520c8e 100644
--- a/include/numeric
+++ b/include/numeric
@@ -157,7 +157,7 @@
         {
             typename iterator_traits<_InputIterator>::value_type __t2(*__first);
             *__result = __t2 - __t1;
-            __t1 = __t2;
+            __t1 = _VSTD::move(__t2);
         }
     }
     return __result;
@@ -177,7 +177,7 @@
         {
             typename iterator_traits<_InputIterator>::value_type __t2(*__first);
             *__result = __binary_op(__t2, __t1);
-            __t1 = __t2;
+            __t1 = _VSTD::move(__t2);
         }
     }
     return __result;