This attempts to make is_nothrow_constructible more robust (and
efficient to compile) by not depending on is_constructible. Instead the
__is_constructible intrinsic is used directly. The helper class
__is_nt_constructible_impl which checks whether the construction is
non-throwing now takes a bool template parameter that is substituted by
the result of the instrinsic. This fixes the reported bug by not using
the already-instantiated (and incorrect) value of std::is_constructible.
I don't think it really fixes the problem in general, because
std::is_nothrow_constructible itself could already have been
instantiated in a context where it gives the wrong result. A proper fix
needs to be done in the compiler.

	PR libstdc++/94033
	* include/std/type_traits (__is_nt_default_constructible_atom): Remove.
	(__is_nt_default_constructible_impl): Remove.
	(__is_nothrow_default_constructible_impl): Remove.
	(__is_nt_constructible_impl): Add bool template parameter. Adjust
	partial specializations.
	(__is_nothrow_constructible_impl): Replace class template with alias
	template.
	(is_nothrow_default_constructible): Derive from alias template
	__is_nothrow_constructible_impl instead of
	__is_nothrow_default_constructible_impl.
	* testsuite/20_util/is_nothrow_constructible/94003.cc: New test.