[libcxx][functional][modular] splices <functional> into modular headers

Differential Revision: https://reviews.llvm.org/D104942

NOKEYCHECK=True
GitOrigin-RevId: 050b064f15ee56ee0b42c9b957a3dd0f32532394
diff --git a/include/set b/include/set
index 0da484b..21ec843 100644
--- a/include/set
+++ b/include/set
@@ -435,6 +435,7 @@
 
 #include <__config>
 #include <__debug>
+#include <__functional/is_transparent.h>
 #include <__node_handle>
 #include <__tree>
 #include <__utility/forward.h>