[time.syn] add constexpr to leap_second comparisons (#4539)

Author: MattStephanson

source/time.tex

@@ -772,27 +772,27 @@
     // \ref{time.zone.leap}, leap second support
     class leap_second;
 
-    bool operator==(const leap_second& x, const leap_second& y);
-    strong_ordering operator<=>(const leap_second& x, const leap_second& y);
+    constexpr bool operator==(const leap_second& x, const leap_second& y);
+    constexpr strong_ordering operator<=>(const leap_second& x, const leap_second& y);
 
     template<class Duration>
-      bool operator==(const leap_second& x, const sys_time<Duration>& y);
+      constexpr bool operator==(const leap_second& x, const sys_time<Duration>& y);
     template<class Duration>
-      bool operator< (const leap_second& x, const sys_time<Duration>& y);
+      constexpr bool operator< (const leap_second& x, const sys_time<Duration>& y);
     template<class Duration>
-      bool operator< (const sys_time<Duration>& x, const leap_second& y);
+      constexpr bool operator< (const sys_time<Duration>& x, const leap_second& y);
     template<class Duration>
-      bool operator> (const leap_second& x, const sys_time<Duration>& y);
+      constexpr bool operator> (const leap_second& x, const sys_time<Duration>& y);
     template<class Duration>
-      bool operator> (const sys_time<Duration>& x, const leap_second& y);
+      constexpr bool operator> (const sys_time<Duration>& x, const leap_second& y);
     template<class Duration>
-      bool operator<=(const leap_second& x, const sys_time<Duration>& y);
+      constexpr bool operator<=(const leap_second& x, const sys_time<Duration>& y);
     template<class Duration>
-      bool operator<=(const sys_time<Duration>& x, const leap_second& y);
+      constexpr bool operator<=(const sys_time<Duration>& x, const leap_second& y);
     template<class Duration>
-      bool operator>=(const leap_second& x, const sys_time<Duration>& y);
+      constexpr bool operator>=(const leap_second& x, const sys_time<Duration>& y);
     template<class Duration>
-      bool operator>=(const sys_time<Duration>& x, const leap_second& y);
+      constexpr bool operator>=(const sys_time<Duration>& x, const leap_second& y);
     template<class Duration>
       requires @\libconcept{three_way_comparable_with}@<sys_seconds, sys_time<Duration>>
       constexpr auto operator<=>(const leap_second& x, const sys_time<Duration>& y);