libstdc++: Make std::move_only_function never valueless in std::variant

libstdc++: Make std::move_only_function never valueless in std::variant

This is not suitable to backport, as it affects the ABI of std::variant
and so isn't appropriate for a release branch.

libstdc++-v3/ChangeLog:

	* include/bits/move_only_function.h (_Never_valueless_alt):
	Define partial specialization for std::move_only_function.