libstdc++: Implement C++26 type checking for std::format args [PR115776]

Implement the changes from P2757R3, which enhance the parse context to
be able to do type checking on format arguments, and to use that to
ensure that args used for width and precisions are integral types.

libstdc++-v3/ChangeLog:

	PR libstdc++/115776
	* include/bits/version.def (format): Update for C++26.
	* include/bits/version.h: Regenerate.
	* include/std/format (basic_format_parse_context): Remove
	default argument from constructor and split into two
	constructors. Make the constructor taking size_t private for
	C++26 and later.
	(basic_format_parse_context::check_dynamic_spec): New member
	function template.
	(basic_format_parse_context::check_dynamic_spec_integral): New
	member function.
	(basic_format_parse_context::check_dynamic_spec_string):
	Likewise.
	(__format::_Spec::_S_parse_width_or_precision): Use
	check_dynamic_spec_integral.
	(__format::__to_arg_t_enum): New helper function.
	(basic_format_arg):  Declare __to_arg_t_enum as friend.
	(__format::_Scanner): Define and use a derived parse context
	type.
	(__format::_Checking_scanner): Make arg types available to parse
	context.
	* testsuite/std/format/functions/format.cc: Check for new values
	of __cpp_lib_format macro.
	* testsuite/std/format/parse_ctx.cc: Check all members of
	basic_format_parse_context.
	* testsuite/std/format/parse_ctx_neg.cc: New test.
	* testsuite/std/format/string.cc: Add more checks for dynamic
	width and precision args.

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