libstdc++: Optimize std::tuple_element and std::tuple_size_v

This reduces the number of class template instantiations needed for code
using tuples, by reusing _Nth_type in tuple_element and specializing
tuple_size_v for tuple, pair and array (and const-qualified versions of
them).

Also define the _Nth_type primary template as a complete type (but with
no nested 'type' member).  This avoids "invalid use of incomplete type"
errors for out-of-range specializations of tuple_element. Those errors
would probably be confusing and unhelpful for users. We already have
a user-friendly static assert in tuple_element itself.

Also ensure that tuple_size_v is available whenever tuple_size is (as
proposed by LWG 3387). We already do that for tuple_element_t.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (tuple_size_v): Define partial
	specializations for std::pair.
	* include/bits/utility.h (_Nth_type): Move definition here
	and define primary template.
	(tuple_size_v): Move definition here.
	* include/std/array (tuple_size_v): Define partial
	specializations for std::array.
	* include/std/tuple (tuple_size_v): Move primary template to
	<bits/utility.h>.  Define partial specializations for
	std::tuple.
	(tuple_element): Change definition to use _Nth_type.
	* include/std/variant (_Nth_type): Move to <bits/utility.h>.
	(variant_alternative, variant): Adjust qualification of
	_Nth_type.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Prune
	additional errors from _Nth_type.

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