libstdc++: Add specializations for some variable templates

This avoids having to instantiate a class template when we can detect
the true cases easily with a partial specialization.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_lvalue_reference_v)
	(is_rvalue_reference_v, is_reference_v, is_const_v)
	(is_volatile_v): Define using partial specializations instead
	of instantiating class templates.