libstdc++: Lift chrono localized formatting to main chrono format loop [PR110739]

libstdc++: Lift chrono localized formatting to main chrono format loop [PR110739]

This patch extract calls to _M_locale_fmt and construction of the struct tm,
from the functions dedicated to each specifier, to main format loop in
_M_format_to functions. This removes duplicated code repeated for specifiers.

To allow _M_locale_fmt to only be called if localized formatting is enabled
('L' is present in chrono-format-spec), we provide a implementations for
locale specific specifiers (%c, %r, %x, %X) that produces the same result
as locale::classic():
 * %c is implemented as separate _M_c method
 * %r is implemented as separate _M_r method
 * %x is implemented together with %D, as they provide same behavior,
 * %X is implemented together with %R as _M_R_X, as both of them do not include
   subseconds.

The handling of subseconds was also extracted to _M_subsecs function that is
used by _M_S and _M_T specifier. The _M_T is now implemented in terms of
_M_R_X (printing time without subseconds) and _M_subs.

The __mod parameter responsible for triggering localized formatting was removed
from methods handling most of specifiers, except:
 * _M_S (for %S) for which it determines if subseconds should be included,
 * _M_z (for %z) for which it determines if ':' is used as separator.

	PR libstdc++/110739

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_use_locale_fmt):
	Define.
	(__formatter_chrono::_M_locale_fmt): Moved to front of the class.
	(__formatter_chrono::_M_format_to): Construct and initialize
	struct tm and call _M_locale_fmt if needed.
	(__formatter_chrono::_M_c_r_x_X): Split into separate methods.
	(__formatter_chrono::_M_c, __formatter_chrono::_M_r): Define.
	(__formatter_chrono::_M_D): Renamed to _M_D_x.
	(__formatter_chrono::_M_D_x): Renamed from _M_D.
	(__formatter_chrono::_M_R_T): Split into _M_R_X and _M_T.
	(__formatter_chrono::_M_R_X): Extracted from _M_R_T.
	(__formatter_chrono::_M_T): Define in terms of _M_R_X and _M_subsecs.
	(__formatter_chrono::_M_subsecs): Extracted from _M_S.
	(__formatter_chrono::_M_S): Replaced __mod with __subs argument,
	removed _M_locale_fmt call, and delegate to _M_subsecs.
	(__formatter_chrono::_M_C_y_Y, __formatter_chrono::_M_d_e)
	(__formatter_chrono::_M_H_I, __formatter_chrono::_M_m)
	(__formatter_chrono::_M_u_w, __formatter_chrono::_M_U_V_W): Remove
	__mod argument and call to _M_locale_fmt.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>