Support inline visibility definition on checking visibility

[r7kamura]

The following patterns have not been supported by RuboCop::Cop::VisibilityHelp#node_visibility:

class A
  private def x
  end
end

class A
  def x
  end
  private :x
end

node_visibility(def_x_node) #=> :public

I think it would be good to support them as well.

The background leading me to this change is as follows:

I am trying to create a Cop that inspects unrecommended visibility of some method definitions. I tried to use the #node_visibility for this implementation, but realized that this would not work as intended in this case if inline style visibility was used. There seemed to be no obvious reason not to support it, so I decided to create this Pull Request.
(FYI: In fact, I'm trying to create a cop that detects some public methods in Rails controller that actually should be private.)

After making the changes, several tests began to fail, so I decided to improve those as well.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Good change! This was indeed just an oversight on our part.