Do not emit certain diagnostics if the fix is not available in the currently used C# version

[SergeyTeplyakov]

For instance, if the application uses C# 7.2, do not emit a warning that a method can be made readonly, because the fix will produce a non-compilable code.

Consider emitting an info level diagnostic that will show that some diagnostics (with a name) will be available if the target language will be changed. This can be a useful hint for the user.