non-member swap for array was mistakenly taking const ref params. Fixed and added test. Thanks to Ben Craig for the catch

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@262866 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/array b/include/array
index 8866eaf..f0350ea 100644
--- a/include/array
+++ b/include/array
@@ -279,7 +279,7 @@
     __is_swappable<_Tp>::value,
     void
 >::type
-swap(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
+swap(array<_Tp, _Size>& __x, array<_Tp, _Size>& __y)
                                   _NOEXCEPT_(__is_nothrow_swappable<_Tp>::value)
 {
     __x.swap(__y);