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

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

NOKEYCHECK=True
GitOrigin-RevId: 050b064f15ee56ee0b42c9b957a3dd0f32532394
diff --git a/include/experimental/functional b/include/experimental/functional
index e189620..e3220e1 100644
--- a/include/experimental/functional
+++ b/include/experimental/functional
@@ -86,6 +86,7 @@
 
 */
 
+#include <__memory/uses_allocator.h>
 #include <experimental/__config>
 #include <functional>
 #include <algorithm>