Fix an incorrect auto-correct for Layout/ClassStructure

[koic]

This PR fixes the following incorrect auto-correct for Layout/ClassStructure when heredoc constant is defined after public method.

% cat example.rb
class Foo
  def foo
  end

  CONSTANT = <<~EOS.freeze
    str
  EOS
end

% bundle exec rubocop --only Layout/ClassStructure -a
(snip)

% cat example.rb
class Foo
  CONSTANT = <<~EOS.freeze
  def foo
  end

    str
  EOS
end

---

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).
• 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.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Looks good! Thanks!