Add check for void defined? and self by Lint/Void cop

[pocke]

MRI checks void defined? and self expression.

See. https://github.com/ruby/ruby/blob/trunk/parse.y#L9571-L9654

However, Lint/Void cop does not check the expressions currently.

 # test.rb
def foo
  self
  defined? Foo
  nil
end

$ rubocop --cache false --only Lint/Void
Inspecting 1 file
.

1 file inspected, no offenses detected

This PR changes the cop to check the expressions.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

In theory we can check for all predicate methods in void context. If someone wrote a predicate with side effects - that predicate's not good anyways. :-)