Ruby: Call-context sensitivity for singleton method calls

[hvitved]

The equivalent of #10358, but for singleton methods instead of instance methods. For example, in

class A
  def self.singleton_method1 x
    sink x
  end

  def self.singleton_method2 x
    singleton_method1 x
  end
end

class B < A
  def self.singleton_method1 x
    non_sink x
  end
end

B.singleton_method2 taint

we will no longer report flow from taint to sink (but still to non_sink).