libstdc++: Use concepts to simplify std::optional base classes

In C++20 mode we can simplify some of the std::optional base class
hierarchy using concepts. We can overload the destructor and copy
constructor and move constructor with a trivial defaulted version and a
constrained non-trivial version. This allows us to remove some class
template partial specializations that were used to conditionally define
those special members as trivial or non-trivial. This should not change
any semantics, but should be less work for the compiler, due to not
needing to match partial specializations, and completely removing one
level of the inheritance hierarchy.

libstdc++-v3/ChangeLog:

	* include/std/optional (_Optional_payload_base::_Storage)
	[C++20]: Define constrained non-trivial destructor.
	(_Optional_payload_base::_Storage<U, false>) [C++20]: Do not
	define partial specialization when primary template has
	constrained destructor.
	(_Optional_base) [C++20]: Define constrained trivial copy and
	move cons and move constructors. Define payload accessors here
	instead of inheriting them from _Optional_base_impl.
	(_Optional_base_impl, _Optional_base<T, false, true>)
	(_Optional_base<T, true, false>, _Optional_base<T, true, true>)
	[C++20]: Do not define.

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