libstdc++: Optimize is_void and is_null_pointer

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.

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