<chrono> formatting: weekday_last, month_weekday, month_weekday_last
#1854
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weekday_last,month_weekday,month_weekday_last._Fill_tm():weekday_indexedandweekday_lastcan share code.month_weekdayandmonth_weekday_lasthave different accessors.year_month_weekdayandyear_month_weekday_last._Is_valid_type():weekday,weekday_indexed, andweekday_lastall support the "weekday types".month_weekdayandmonth_weekday_lastsupport "month types" and "weekday types". (As mentioned above, their accessors are actuallyweekday_indexed()andweekday_last(), but it seemed pointless to have separate cases to "recurse" into theweekday_indexedandweekday_lasttypes, when the answer is always the same.)static_assertto"should be unreachable", which is the pattern that we use elsewhere._Fill_tm_formatter.P0355R7_calendars_and_time_zones_formatting/test.cpp:charTtoCharTfor consistency (this is needed by theSTRmacro, if it were ever used in these functions).empty_braces_helper()to test bothformat("{}")andoperator<<. This should supersedestream_helper()but I'm not making that change here.year_month_day_last,year_month_weekday, andyear_month_weekday_lastnow that the necessary formatters are available.