[libc++] [P1614] Implement std::compare_three_way.

Differential Revision: https://reviews.llvm.org/D110735

NOKEYCHECK=True
GitOrigin-RevId: 3df094d31eac753687c7963738bb847e6d3794e9
diff --git a/include/functional b/include/functional
index 3cff866..53a5f2b 100644
--- a/include/functional
+++ b/include/functional
@@ -135,6 +135,9 @@
     bool operator()(const T& x, const T& y) const;
 };
 
+// [comparisons.three.way], class compare_three_way
+struct compare_three_way;
+
 template <class T> // <class T=void> in C++14
 struct logical_and {
     bool operator()(const T& x, const T& y) const;
@@ -488,6 +491,7 @@
 */
 
 #include <__algorithm/search.h>
+#include <__compare/compare_three_way.h>
 #include <__config>
 #include <__debug>
 #include <__functional/binary_function.h> // TODO: deprecate