[libc++] [compare] Named comparison functions, is_eq etc.

Some of these were previously half-implemented in "ordering.h";
now they're all implemented, and tested.
Note that `constexpr` functions are implicitly `inline`, so the
standard wording omits `inline` on these; but Louis and I agree
that that's surprising and it's better to be explicit about it.

Differential Revision: https://reviews.llvm.org/D110515

NOKEYCHECK=True
GitOrigin-RevId: 969359e3b86b6cd238bcb48cdaeb9be5dbeddb4b
diff --git a/include/module.modulemap b/include/module.modulemap
index 89ac506..f85e9e3 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -373,6 +373,7 @@
     module __compare {
       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 is_eq                      { private header "__compare/is_eq.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"       }