Instead of defining these in terms of a helper class template
and the relatively expensive __remove_cv_t, just declare four
explicit specializations of the main template, one for each choice
of cv-quals.
libstdc++-v3/ChangeLog:
* include/std/type_traits (__is_void_helper): Remove.
(is_void): Make the primary template derive from false_type,
and define four explicit specializations that derive from
true_type.
(__is_null_pointer_helper, is_null_pointer): Likewise.