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

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

NOKEYCHECK=True
GitOrigin-RevId: 050b064f15ee56ee0b42c9b957a3dd0f32532394
diff --git a/include/future b/include/future
index 5d2732b..349e6af 100644
--- a/include/future
+++ b/include/future
@@ -364,6 +364,8 @@
 #include <__availability>
 #include <__config>
 #include <__debug>
+#include <__memory/allocator_arg_t.h>
+#include <__memory/uses_allocator.h>
 #include <__utility/__decay_copy.h>
 #include <__utility/forward.h>
 #include <chrono>