Do not warn for overridden messages

[sailro]

Fixes #187

Checklist

• I have read the Contribution Guide ;
• There is an approved issue describing the change when contributing a new analyzer or suppressor ;
• I have added tests that prove my fix is effective or that my feature works ;
• I have added necessary documentation (if appropriate) ;

Short description of what this resolves:

We should not warn for overridden messages, given they will use the scope of the member definition. This rule is only interesting for regular messages. So check the IsOverride on the method symbol.