libstdc++: Fix localized D_T_FMT %c formatting for <chrono> [PR117214]

libstdc++: Fix localized D_T_FMT %c formatting for <chrono> [PR117214]

Formatting a time point with %c was implemented by calling
std::vprint_to with format string constructed from locale's D_T_FMT
string, but in some locales this string contains strftime specifiers
which are not valid for chrono-specs, e.g. %l. So just use _M_locale_fmt
to avoid this problem.

libstdc++-v3/ChangeLog:

	PR libstdc++/117214
	* include/bits/chrono_io.h (__formatter_chrono::_M_c): Use
	_M_locale_fmt to format %c time point.
	* testsuite/std/time/format/pr117214.cc: New test.

Signed-off-by: XU Kailiang <xu2k3l4@outlook.com>
Co-authored-by: Jonathan Wakely <jwakely@redhat.com>