libstdc++: Use runtime format for internal format calls in chrono [PR110739]

libstdc++: Use runtime format for internal format calls in chrono [PR110739]

This patch adjust all internal std::format call inside of __formatter_chrono,
to use runtime format string and thus avoid compile time checking of validity
of the format string. Majority of cases are covered by calling newly introduced
_S_empty_fs() function that returns __Runtime_format_string containing
_S_empty_spec, instead of passing later directly.

In case of _M_j we use _S_str_d3 function (extracted from _S_str_d2), eliminating
call to std::format outside of unlikely scenario in which day of year is greater
than 1000 (this may happen for year_month_day with month greater than 12). In
consequence, outside of handling subseconds, we no longer delegate to std::format
or construct temporary strings, when formatting chrono types with ok() values.

	PR libstdc++/110739

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_S_empty_fs): Define.
	(__formatter_chrono::_S_str_d2): Use _S_str_d3 for 3+ digits and
	place allways_inline attribute after comment.
	(__formatter_chrono::_S_str_d3): Extracted from _S_str_d2.
	(__formatter_chrono::_M_H_I, __formatter_chrono::_M_R_X): Replace
	_S_empty_spec with _S_empty_fs().
	(__formatter_chrono::_M_j): Likewise and use _S_str_d3 in common
	case.
	(__format::operator-(_ChronoParts, _ChronoParts))
	(__format::operator-=(_ChronoParts, _ChronoParts))
	(__formatter_chrono::_S_fill_two_digits)
	(__formatter_chrono::_S_str_d1): Place always_inline attribute
	after comment.