libstdc++: Simplify C++20 implementation of std::variant

For C++20 the __detail::__variant::_Uninitialized primary template can
be used for all types, because _Variant_union can have a non-trivially
destructible union member in C++20, and the constrained user-provided
destructor will ensure we don't destroy inactive objects.

Since we always use the primary template for C++20, we don't need the
_Uninitialized::_M_get accessors to abstract the difference between the
primary template and the partial specialization. That allows us to
simplify __get_n for C++20 too.

Also improve the comments that explain the uses of _Uninitialized and
when/why _Variant_union needs a user-provided destructor.

libstdc++-v3/ChangeLog:

	* include/std/variant [C++20] (_Uninitialized): Always use the
	primary template.
	[C++20] (__get_n): Access the _M_storage member directly.

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