libstdc++: Add missing constexpr to std::variant (P2231R1)

This implements the changes in P2231R1 which make std::variant fully
constexpr in C++20.

We need to replace placement new with std::construct_at, but that isn't
defined for C++17. Use std::_Construct instead, which forwards to
std::construct_at in C++20 mode (since the related changes to make
std::optional fully constexpr, in r12-4389).

We also need to replace the untyped char buffer in _Uninitialized with a
union, which can be accessed in constexpr functions. But the union needs
to have a non-trivial destructor if its variant type is non-trivial,
which means that the _Variadic_union also needs a non-trivial
destructor. This adds a constrained partial specialization of
_Variadic_union for the C++20-only case where a non-trivial destructor
is needed.

We can't use concepts to constrain the specialization (or the primary
template's destructor) in C++17, so retain the untyped char buffer
solution for C++17 mode.


	* include/std/variant (__cpp_lib_variant): Update value for
	(__variant_cast, __variant_construct): Add constexpr for C++20.
	(__variant_construct_single, __construct_by_index) Likewise. Use
	std::_Construct instead of placement new.
	(_Uninitialized<T, false>) [__cplusplus >= 202002]: Replace
	buffer with a union and define a destructor.
	(_Variadic_union) [__cplusplus >= 202002]: Add a specialization
	for non-trivial destruction.
	(_Variant_storage::__index_of): New helper variable template.
	(_Variant_storage::~_Variant_storage()): Add constexpr.
	(_Variant_storage::_M_reset()): Likewise.
	(_Copy_ctor_base, _Move_ctor_base): Likewise.
	(_Copy_assign_base, _Move_assign_base): Likewise.
	(variant, swap): Likewise.
	* include/std/version (__cpp_lib_variant): Update value for
	* testsuite/20_util/optional/ Check for exact value
	in C++17.
	* testsuite/20_util/variant/ Increase timeout for
	C++20 mode.
	* testsuite/20_util/variant/ New test.
	* testsuite/20_util/variant/ New test.
6 files changed