Merge to upstream r222492.

Change-Id: I6a0a6e90d217a69531ec3bb5ca0a367f39f4a1da
diff --git a/include/unordered_set b/include/unordered_set
index fd378fa..d06629f 100644
--- a/include/unordered_set
+++ b/include/unordered_set
@@ -325,6 +325,8 @@
 #include <__hash_table>
 #include <functional>
 
+#include <__debug>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif