Add support for `private_class_method` in the indexer

[vinistock]

Very similar to #2653 and how we handle private constants. We need to start taking private_class_method into consideration in the indexer.

When not using the class << self syntax, the private method has no impact whatsoever on the visibility of methods and the only way to mark them as private is by invoking private_class_method.

class Foo
  private

  # this method is public
  def self.bar
  end

  # this makes it private
  private_class_method(:bar)
end

Essentially, we need to:

• Add a new branch here to handle the private_class_method
• Grab the entry that was already inserted in the index for that method and owner combination
• Mark the method as private

[kcdragon]

I'm going to take a look at this at the RubyConf hack day.