libstdc++: Optimize std::variant traits and improve diagnostics

By defining additional partial specializations of _Nth_type we can
reduce the number of recursive instantiations needed to get from N to 0.
We can also use _Nth_type in variant_alternative, to take advantage of
that new optimization.

By adding a static_assert to variant_alternative we get a nicer error
than 'invalid use of incomplete type'.

By defining partial specializations of std::variant_size_v for the
common case we can avoid instantiating the std::variant_size class
template.

The __tuple_count class template and __tuple_count_v variable template
can be simplified to a single variable template, __count.

By adding a deleted constructor to the _Variant_union primary template
we can (very slightly) improve diagnostics for invalid attempts to
construct a std::variant with an out-of-range index. Instead of a
confusing error about "too many initializers for ..." we get a call to a
deleted function.

By using _Nth_type instead of variant_alternative (for cv-unqualified
variant types) we avoid instantiating variant_alternative.

By adding deleted overloads of variant::emplace we get better
diagnostics for emplace<invalid-index> or emplace<invalid-type>. Instead
of getting errors explaining why each of the four overloads wasn't
valid, we just get one error about calling a deleted function.

libstdc++-v3/ChangeLog:

	* include/std/variant (_Nth_type): Define partial
	specializations to reduce number of instantiations.
	(variant_size_v): Define partial specializations to avoid
	instantiations.
	(variant_alternative): Use _Nth_type. Add static assert.
	(__tuple_count, __tuple_count_v): Replace with ...
	(__count): New variable template.
	(_Variant_union): Add deleted constructor.
	(variant::__to_type): Use _Nth_type.
	(variant::emplace): Use _Nth_type. Add deleted overloads for
	invalid types and indices.

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