[libc++][spaceship] Implement std::pair::operator<=>

Implements parts of P1614, including synth-three-way and three way comparison for std::pair.

Reviewed By: #libc, Quuxplusone, Mordante

Differential Revision: https://reviews.llvm.org/D107721

NOKEYCHECK=True
GitOrigin-RevId: f4abdb0c074bd7b7b89d9828d449078d16d0e1c3
diff --git a/include/module.modulemap b/include/module.modulemap
index 7a0d673..89ac506 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -374,6 +374,7 @@
       module common_comparison_category { private header "__compare/common_comparison_category.h" }
       module compare_three_way_result   { private header "__compare/compare_three_way_result.h"   }
       module ordering                   { private header "__compare/ordering.h"                   }
+      module synth_three_way            { private header "__compare/synth_three_way.h"            }
       module three_way_comparable       { private header "__compare/three_way_comparable.h"       }
     }
   }