PR libstdc++/87431 optimise valueless_by_exception()

PR libstdc++/87431 optimise valueless_by_exception()

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

	PR libstdc++/87431
	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
	Avoid runtime test when all alternatives are scalars and so cannot
	throw during initialization.

From-SVN: r264574