[libc++][TZDB] Implements zoned_seconds typedef. (llvm#95141)

Implements parts of:
- P0355 Extending to chrono Calendars and Time Zones

Author: mordante

libcxx/include/__chrono/zoned_time.h

@@ -205,6 +205,8 @@ template <class _Duration, class _TimeZonePtrOrName, class TimeZonePtr2>
 zoned_time(_TimeZonePtrOrName&&, zoned_time<_Duration, TimeZonePtr2>, choose = choose::earliest)
     -> zoned_time<common_type_t<_Duration, seconds>, __time_zone_representation<_TimeZonePtrOrName>>;
 
+using zoned_seconds = zoned_time<seconds>;
+
 template <class _Duration1, class _Duration2, class _TimeZonePtr>
 _LIBCPP_HIDE_FROM_ABI bool
 operator==(const zoned_time<_Duration1, _TimeZonePtr>& __lhs, const zoned_time<_Duration2, _TimeZonePtr>& __rhs) {

libcxx/include/chrono

@@ -793,6 +793,8 @@ template<class T> struct zoned_traits;
 template<class Duration, class TimeZonePtr = const time_zone*>                   // C++20
 class zoned_time;
 
+using zoned_seconds = zoned_time<seconds>;                                       // C++20
+
 template<class Duration1, class Duration2, class TimeZonePtr>                    // C++20
   bool operator==(const zoned_time<Duration1, TimeZonePtr>& x,
                   const zoned_time<Duration2, TimeZonePtr>& y);

libcxx/modules/std/chrono.inc

@@ -233,9 +233,7 @@ export namespace std {
     // [time.zone.zonedtime], class template zoned_time
     using std::chrono::zoned_time;
 
-#    if 0
     using std::chrono::zoned_seconds;
-#    endif // if 0
 
     // [time.zone.leap], leap second support
     using std::chrono::leap_second;

libcxx/test/std/time/time.zone/time.zone.zonedtime/types.compile.pass.cpp

@@ -23,6 +23,8 @@
 //    zoned_time& operator=(const zoned_time&) = default;
 //
 //  };
+//
+// using zoned_seconds = zoned_time<seconds>;
 
 #include <chrono>
 #include <concepts>
@@ -61,3 +63,6 @@ static_assert(!std::is_copy_constructible_v<std::chrono::zoned_time< std::chrono
 static_assert(!std::is_move_constructible_v<std::chrono::zoned_time< std::chrono::days, std::unique_ptr<int>>>);
 static_assert(!std::is_copy_assignable_v<std::chrono::zoned_time< std::chrono::days, std::unique_ptr<int>>>);
 static_assert(!std::is_move_assignable_v<std::chrono::zoned_time< std::chrono::days, std::unique_ptr<int>>>);
+
+// using zoned_seconds = zoned_time<seconds>;
+static_assert(std::same_as<std::chrono::zoned_seconds, std::chrono::zoned_time<std::chrono::seconds>>);