libstdc++: Enforce requirements on template argument of std::optional

The standard does not allow std::optional<T&>, std::optional<T[1]>,
std::optional<T()> etc. and although we do give errors, they come from
down inside the internals of std::optional. We could improve the static
assertions at the top of the class so that users get a more precise
diagnostic:

optional:721:21: error: static assertion failed
721 |       static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);

libstdc++-v3/ChangeLog:

	* include/std/optional (optional): Adjust static assertion to
	reject arrays and functions as well as references.
	* testsuite/20_util/optional/requirements_neg.cc: New test.

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