libstdc++: Use variable template to fix -fconcepts-ts error [PR113366]

libstdc++: Use variable template to fix -fconcepts-ts error [PR113366]

There's an error for -fconcepts-ts due to using a concept where a bool
NTTP is required, which is fixed by using the vraiable template that
already exists in the class scope.

This doesn't fix the problem with -fconcepts-ts as changes to the
placement of attributes is also needed.

libstdc++-v3/ChangeLog:

	PR testsuite/113366
	* include/std/format (basic_format_arg): Use __formattable
	variable template instead of __format::__formattable_with
	concept.

(cherry picked from commit 6c703b4eb68cbc32de1d62e5b573cb1b9857af29)