libstdc++: Constrain formatters for chrono types [PR119517]

libstdc++: Constrain formatters for chrono types [PR119517]

The formatters for chrono types defined the parse/format methods
as accepting unconstrained types, this in combination with lack
of constrain on _CharT lead to them falsely satisfying formattable
requirements for any type used as character.

This patch adjust the fromatter<T, CharT>::parse signature to:
 constexpr typename basic_format_parse_context<_CharT>::iterator
 parse(basic_format_parse_context<_CharT>& __pc);
And formatter<T, CharT>::format to:
 template<typename _Out>
   typename basic_format_context<_Out, _CharT>::iterator
   format(const T& __t,
          basic_format_context<_Out, _CharT>& __fc) const;

Furthermore we _CharT with __format::__char (char or wchar_t),

	PR libstdc++/119517

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (formatter):
	Add __format::__char for _CharT and adjust parse and format
	method signatures.
	* testsuite/std/time/format/pr119517.cc: New test.

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