libstdc++: Use basic_format_parse_context<_CharT> in internal chrono formatters.

libstdc++: Use basic_format_parse_context<_CharT> in internal chrono formatters.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_parse):
	Replace _ParseContext with basic_format_parse_context<_CharT> and
	make it non-template.
	(__formatter_duration::_M_parse): Replace _ParseContext with
	basic_format_parse_context<_CharT> and remove unused default
	argument.

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