Add is_swappable/is_nothrow_swappable traits

llvm-svn: 267079
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f07dd8d0a925dd8cbde7bb6198c1ba92446110ea
diff --git a/include/algorithm b/include/algorithm
index 2ea1dae..7a6db7a 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -630,7 +630,7 @@
 #include <initializer_list>
 #include <type_traits>
 #include <cstring>
-#include <utility>
+#include <utility> // needed to provide swap_ranges.
 #include <memory>
 #include <iterator>
 #include <cstddef>