libstdc++: Remove std::basic_format_args default constructor (LWG 4106)

libstdc++: Remove std::basic_format_args default constructor (LWG 4106)

There's no valid use case for default constructing this type, so the
committee approved removing the default constructor.

libstdc++-v3/ChangeLog:

	* include/std/format (basic_format_args): Remove default
	constructor, as per LWG 4106.
	* testsuite/std/format/arguments/args.cc: Check it isn't default
	constructible.