libstdc++: Reduce std::variant template instantiation depth

libstdc++: Reduce std::variant template instantiation depth

The recursively defined constraints on _Variadic_union's user-defined
destructor (used for maintaining trivial destructibility of the variant
iff all of its alternatives are) turn out to require a template
instantiation depth of 3x the number of variants in C++20 mode, with the
instantiation stack looking like

  ...
  _Variadic_union<B, C, ...>
  std::is_trivially_destructible_v<_Variadic_union<B, C, ...>>
  _Variadic_union<A, B, C, ...>::~_Variadic_union()
  _Variadic_union<A, B, C, ...>
  ...

Ideally the template depth should be ~equal to the number of variants
(plus a constant).  Luckily it seems we don't need to compute trivial
destructibility of the alternatives at all from _Variadic_union, since
its only user _Variant_storage already has that information.  To that
end this patch removes these recursive constraints and instead passes
this information down from _Variant_storage.  After this patch, the
template instantiation depth for 87619.cc in C++20 mode is ~270 instead
of ~780.

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::_Variadic_union):
	Add bool __trivially_destructible template parameter.
	(__detail::__variant::_Variadic_union::~_Variadic_union):
	Use __trivially_destructible in constraints instead.
	(__detail::__variant::_Variant_storage): Pass
	__trivially_destructible value to _Variadic_union.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>