libstdc++: Fix std::format_kind primary template for Clang [PR120190]

libstdc++: Fix std::format_kind primary template for Clang [PR120190]

Although Clang trunk has been adjusted to handle our std::format_kind
definition (because they need to be able to compile the GCC 15.1.0
release), it's probably better to not rely on something that they might
start diagnosing again in future.

Define the primary template in terms of an immediately invoked function
expression, so that we can put a static_assert(false) in the body.

libstdc++-v3/ChangeLog:

	PR libstdc++/120190
	* include/std/format (format_kind): Adjust primary template to
	not depend on itself.
	* testsuite/std/format/ranges/format_kind_neg.cc: Adjust
	expected errors. Check more invalid specializations.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Daniel Krügler <daniel.kruegler@gmail.com>