Improve static_assert messages for std::variant

Improve static_assert messages for std::variant

Also fix a warning with -Wunused-parameter -Wsystem-headers.

	* include/std/variant (get<T>, get<N>, get_if<N>, get_if<T>)
	(variant::emplace): Change static_assert messages from "should be"
	to "must be".
	(hash<monostate>::operator()): Remove name of unused parameter.

From-SVN: r272188