libstdc++: Implement correct locale-specific chrono formatting [PR110719]

This fixes some TODOs in the C++20 <chrono> format support, where the
locale-specific output was incorrect or unimplemented. The approach
taken here is to either use the formatting locale's std::time_put facet
to do the formatting, or to remove subsecond precision from time points
so that locale-specific formats don't print fractional seconds. This
ensures that we are consistent with what the std::time_put facet would
print (which never includes fractional seconds) even if we actually
reimplement the formatting by hand instead of using the facet.

This also fixes a misplaced statement that allowed modifiers for %Z
which should have been on %z instead. There was also some ill-formed
code in an untested branch for formatting time zone names to wide
characters. A new test for zoned_time I/O has been added to exercise
that code properly.

libstdc++-v3/ChangeLog:

	PR libstdc++/110719
	* include/bits/chrono_io.h (__formatter_chrono::_M_parse): Fix
	allowed modifiers for %z and %Z. Fix -Wparentheses and
	-Wnarrowing warnings.
	(__formatter_chrono::_M_format): Call new functions for %d, %e,
	%H, %I, %m and %M.
	(__formatter_chrono::_M_c): Use _S_floor_seconds to remove
	subsecond precision.
	(__formatter_chrono::_M_C_y_Y): Use _M_locale_fmt to handle
	modifiers.
	(__formatter_chrono::_M_e): Replace with _M_d_e and use
	_M_locale_fmt.
	(__formatter_chrono::_M_I): Replace with _M_H_I and use
	_M_locale_fmt.
	(__formatter_chrono::_M_m): New function.
	(__formatter_chrono::_M_M): New function.
	(__formatter_chrono::_M_r): Use _M_locale_fmt.
	(__formatter_chrono::_M_S): Likewise.
	(__formatter_chrono::_M_u_w): Likewise.
	(__formatter_chrono::_M_U_V_W): Likewise.
	(__formatter_chrono::_M_X): Use _S_floor_seconds.
	(__formatter_chrono::_M_Z): Fix untested branch for wchar_t.
	(__formatter_chrono::_S_altnum): Remove function.
	(__formatter_chrono::_S_dd_zero_fill): Remove function.
	(__formatter_chrono::_S_floor_seconds): New function.
	(__formatter_chrono::_M_locale_fmt): New function.
	* testsuite/std/time/clock/system/io.cc: Adjust expected output
	for locale-specific formats and check modified formats.
	* testsuite/std/time/clock/utc/io.cc: Likewise.
	* testsuite/std/time/zoned_time/io.cc: New test.

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