[libc++][spaceship] P1614R2: Removed `operator!=` from `weekday`s

[H-G-Hristov]

Summary

Implements parts of P1614R2. Removed operator!= from:

• weekday
• weekday_indexed
• weekday_last
• month_weekday
• month_weekday_last
• year_month_weekday
• year_month_weekday_last

Note these operators were added and removed in C++20.

[H-G-Hristov]

Rebased + implementation

[Mordante]

LGTM! I added a note to the commit message, please land with the new text.

libcxx/docs/Status/SpaceshipProjects.csv

 | `chrono::year_month_day_last <https://reviews.llvm.org/D129887>`_",None,Mark de Wever,|Complete|
 "| `[time.cal.wd] <https://wg21.link/time.cal.wd>`_
 | `[time.cal.wdidx] <https://wg21.link/time.cal.wdidx>`_
 | `[time.cal.wdlast] <https://wg21.link/time.cal.wdlast>`_
 | `[time.cal.mwd] <https://wg21.link/time.cal.mwd>`_
 | `[time.cal.mwdlast] <https://wg21.link/time.cal.mwdlast>`_
 | `[time.cal.ymwd] <https://wg21.link/time.cal.ymwd>`_
 | `[time.cal.ymwdlast] <https://wg21.link/time.cal.ymwdlast>`_","| `weekday`
-| `weekday_indexed`
-| `weekday_last`
-| `month_weekday`
-| `month_weekday_last`
-| `year_month_weekday`
-| `year_month_weekday_last`",None,Unassigned,|Not Started|
+| `weekday_indexed <https://reviews.llvm.org/D130859>`_
+| `weekday_last <https://reviews.llvm.org/D130859>`_
+| `month_weekday <https://reviews.llvm.org/D130859>`_
+| `month_weekday_last <https://reviews.llvm.org/D130859>`_
+| `year_month_weekday <https://reviews.llvm.org/D130859>`_
+| `year_month_weekday_last`",None,Hristo Hristov,|Complete|
 "| `[time.zone.nonmembers] <https://wg21.link/time.zone.nonmembers>`_
 | `[time.zone.zonedtime.nonmembers] <https://wg21.link/time.zone.zonedtime.nonmembers>`_
 | `[time.zone.leap.nonmembers] <https://wg21.link/time.zone.leap.nonmembers>`_
 | `[time.zone.link.nonmembers] <https://wg21.link/time.zone.link.nonmembers>`_","| chrono::time_zone
 | chrono::leap_second
 | chrono::time_zone_link",A ``<chrono>`` implementation,Unassigned,|Not Started|
 - `5.13 Clause 28: Localization library <https://wg21.link/p1614r2#clause-28-localization-library>`_,,,,
 "| `[locale] <https://wg21.link/locale>`_

libcxx/include/chrono

 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const year& y);
 
 // 25.8.6, class weekday    // C++20
 class weekday;
 
 constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
-constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;
+constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;                 // removed in C++20
 constexpr weekday operator+(const weekday& x, const days&    y) noexcept;
 constexpr weekday operator+(const days&    x, const weekday& y) noexcept;
 constexpr weekday operator-(const weekday& x, const days&    y) noexcept;
 constexpr days    operator-(const weekday& x, const weekday& y) noexcept;
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
 
 // 25.8.7, class weekday_indexed    // C++20
 
 class weekday_indexed;
 constexpr bool operator==(const weekday_indexed& x, const weekday_indexed& y) noexcept;
-constexpr bool operator!=(const weekday_indexed& x, const weekday_indexed& y) noexcept;
+constexpr bool operator!=(const weekday_indexed& x, const weekday_indexed& y) noexcept; // removed in C++20
 
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const weekday_indexed& wdi);
 
 // 25.8.8, class weekday_last    // C++20
 class weekday_last;
 
 constexpr bool operator==(const weekday_last& x, const weekday_last& y) noexcept;
-constexpr bool operator!=(const weekday_last& x, const weekday_last& y) noexcept;
+constexpr bool operator!=(const weekday_last& x, const weekday_last& y) noexcept;       // removed in C++20
 
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const weekday_last& wdl);
 
 // 25.8.9, class month_day    // C++20
 class month_day;
 

 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const month_day_last& mdl);
 
 // 25.8.11, class month_weekday    // C++20
 class month_weekday;
 
 constexpr bool operator==(const month_weekday& x, const month_weekday& y) noexcept;
-constexpr bool operator!=(const month_weekday& x, const month_weekday& y) noexcept;
+constexpr bool operator!=(const month_weekday& x, const month_weekday& y) noexcept;           // removed in C++20
 
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const month_weekday& mwd);
 
 // 25.8.12, class month_weekday_last    // C++20
 class month_weekday_last;
 
 constexpr bool operator==(const month_weekday_last& x, const month_weekday_last& y) noexcept;
-constexpr bool operator!=(const month_weekday_last& x, const month_weekday_last& y) noexcept;
+constexpr bool operator!=(const month_weekday_last& x, const month_weekday_last& y) noexcept; // removed in C++20
 
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const month_weekday_last& mwdl);
 
 
 // 25.8.13, class year_month    // C++20
 class year_month;
basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const year_month_day_last& ymdl);
 
 // 25.8.16, class year_month_weekday    // C++20
 class year_month_weekday;
 
 constexpr bool operator==(const year_month_weekday& x,
                           const year_month_weekday& y) noexcept;
 constexpr bool operator!=(const year_month_weekday& x,
-                          const year_month_weekday& y) noexcept;
+                          const year_month_weekday& y) noexcept;                  // removed in C++20
 
 constexpr year_month_weekday
   operator+(const year_month_weekday& ymwd, const months& dm) noexcept;
 constexpr year_month_weekday
   operator+(const months& dm, const year_month_weekday& ymwd) noexcept;
 constexpr year_month_weekday
   operator+(const year_month_weekday& ymwd, const years& dy) noexcept;
 constexpr year_month_weekday
   operator+(const years& dy, const year_month_weekday& ymwd) noexcept;
 constexpr year_month_weekday
   operator-(const year_month_weekday& ymwd, const months& dm) noexcept;
 constexpr year_month_weekday
   operator-(const year_month_weekday& ymwd, const years& dy) noexcept;
 
 template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const year_month_weekday& ymwd);
 
 // 25.8.17, class year_month_weekday_last    // C++20
 class year_month_weekday_last;
 
 constexpr bool operator==(const year_month_weekday_last& x,
                           const year_month_weekday_last& y) noexcept;
 constexpr bool operator!=(const year_month_weekday_last& x,
-                          const year_month_weekday_last& y) noexcept;
+                          const year_month_weekday_last& y) noexcept;             // removed in C++20
 constexpr year_month_weekday_last
   operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
 constexpr year_month_weekday_last
   operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;
 constexpr year_month_weekday_last
   operator+(const year_month_weekday_last& ymwdl, const years& dy) noexcept;
 constexpr year_month_weekday_last
   operator+(const years& dy, const year_month_weekday_last& ymwdl) noexcept;