Use diagnostics to inform that static methods in classes are not mocked

[swift-student]

Static methods cannot be mocked in classes because the mock uses a subclass and static methods cannot be overridden. We could use diagnostic message with a severity of remark for this. I'm not sure yet how to view remarks other than they tend to show briefly when compiling and then go away, though maybe that is just an Xcode bug. We could allow upgrading them to warnings though using a parameter in the macro.