Overlap with VS 15.5.1 IDE refactorings

If a private method does not have the `private` keyword, in VS 15.5.1, two refactoring with the same behavior appear:

* Add default access modifier (from Roslynator)
* Add accessibility modifier (from VS)

For refactorings that are redundant with those in the IDE, Roslynator should disable its or the IDE's version of the refactoring.

Reproduce the overlap in Visual Studio 15.5.1 with a private method that lacks the private keyword, then compare Roslynator's "Add default access modifier" with Visual Studio's "Add accessibility modifier." Done means deciding which redundant refactoring should be disabled and confirming that only one action remains available.

Written by the indexing model from the issue text.