[libc++] Make __weekday_from_days private in weekday

[jloser]

Summary

weekday has a static member function __weekday_from_days which is
not part of the mandated public interface of weeekday according to the
standard. Since it is only used internally in the constructors of
weekday, let's make it private.

[Mordante]

LGTM!

libcxx/include/chrono

 { return static_cast<int>(min()) <= __y && __y <= static_cast<int>(max()); }
 
 class weekday_indexed;
 class weekday_last;
 
 class weekday {
 private:
     unsigned char __wd;
+    static constexpr unsigned char __weekday_from_days(int __days) noexcept;
 public:
   weekday() = default;
   inline explicit constexpr weekday(unsigned __val) noexcept : __wd(static_cast<unsigned char>(__val == 7 ? 0 : __val)) {}
   inline constexpr          weekday(const sys_days& __sysd) noexcept
           : __wd(__weekday_from_days(__sysd.time_since_epoch().count())) {}
   inline explicit constexpr weekday(const local_days& __locd) noexcept
           : __wd(__weekday_from_days(__locd.time_since_epoch().count())) {}
 
   inline constexpr weekday& operator++()    noexcept { __wd = (__wd == 6 ? 0 : __wd + 1); return *this; }
   inline constexpr weekday  operator++(int) noexcept { weekday __tmp = *this; ++(*this); return __tmp; }
   inline constexpr weekday& operator--()    noexcept { __wd = (__wd == 0 ? 6 : __wd - 1); return *this; }
   inline constexpr weekday  operator--(int) noexcept { weekday __tmp = *this; --(*this); return __tmp; }
          constexpr weekday& operator+=(const days& __dd) noexcept;
          constexpr weekday& operator-=(const days& __dd) noexcept;
   inline constexpr unsigned c_encoding()   const noexcept { return __wd; }
   inline constexpr unsigned iso_encoding() const noexcept { return __wd == 0u ? 7 : __wd; }
   inline constexpr bool ok() const noexcept { return __wd <= 6; }
          constexpr weekday_indexed operator[](unsigned __index) const noexcept;
          constexpr weekday_last    operator[](last_spec) const noexcept;
-
-  // TODO: Make private?
-  static constexpr unsigned char __weekday_from_days(int __days) noexcept;
 };
 
 
 // https://howardhinnant.github.io/date_algorithms.html#weekday_from_days
 inline constexpr
 unsigned char weekday::__weekday_from_days(int __days) noexcept
 {
     return static_cast<unsigned char>(