This change simplifies visitation for variants, by using INVOKE<R> for
the visit<R> form, and explicitly specifying the tag types for raw
visitation, instead of inferring them from the return types of the
lambda functions used as visitors.

	* include/std/variant (__visit_with_index): Remove typedef.
	(__deduce_visit_result): New tag type.
	(__raw_visit, __raw_idx_visit): New helper functions for "raw"
	visitation of possibly-valueless variants, forwarding to __do_visit
	with the relevant tag type.
	(_Variant_storage<false, _Types...>::_M_reset_impl): Use __raw_visit
	and make lambda return void.
	(__variant_construct): Likewise.
	(_Copy_assign_base::operator=, _Move_assign_base::operator=): Use
	__raw_idx_visit and make lambda return void.
	(_Multi_array::__untag_result): Add metafunction to check the function
	pointer type for a tag type that dictates the kind of visitation.
	(_Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>):
	Use decltype(auto) instead of tagged function pointer type.
	(__gen_vtable_impl): Remove bool non-type parameter and unused
	_Variant_tuple parameter.
	(__gen_vtable_impl::__visit_invoke_impl): Remove.
	(__gen_vtable_impl::__do_visit_invoke): Remove.
	(__gen_vtable_impl::__do_visit_invoke_r): Remove.
	(__gen_vtable_impl::__visit_invoke): Use if-constexpr and __invoke_r
	for the visit<R> case, rather than dispatching to separate functions.
	(_VARIANT_RELATION_FUNCTION_TEMPLATE): Use __raw_idx_visit and make
	lambda return void.
	(variant::swap): Likewise.
	(__do_visit): Replace two non-type template parameters with a single
	type parameter, so that the caller must specify the visitor's return
	type (or one of the tag types).
	(visit): Deduce a return type from the visitor and use the
	__deduce_visit_result tag to enforce that all overloads return the
	same type.
	(visit<R>): Call __do_visit<R> with explicit result type.
	(__variant_hash_call_base_impl::operator()): Use __raw_visit and make
	lambda return void.

From-SVN: r271182