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.