[libc++][NFC] Add a missing include in pre-C++14 language modes.

var-const · var-const · commit f51c91532e24 · 2022-11-17T17:15:41.000-08:00
diff --git a/libcxx/include/__memory/swap_allocator.h b/libcxx/include/__memory/swap_allocator.h
@@ -12,6 +12,7 @@
 #include <__config>
 #include <__memory/allocator_traits.h>
 #include <__type_traits/integral_constant.h>
+#include <__type_traits/is_swappable.h>
 #include <__utility/swap.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
