Avoid redundant runtime checks in std::visit

Avoid redundant runtime checks in std::visit

Calling std::get will check some static assertions and also do a runtime
check for a valid index before calling __detail::__variant::__get. The
std::visit function already handles the case where any variant has an
invalid index, so __get can be used directly in __visit_invoke.

	* include/std/variant (__gen_vtable_impl::__visit_invoke): Call __get
	directly instead of get, as caller ensures correct index is used.
	(holds_alternative, get, get_if): Remove redundant inline specifiers.
	(_VARIANT_RELATION_FUNCTION_TEMPLATE): Likewise.

From-SVN: r264786