Fix self union type checking

[marcotc]

This fixes the case where a method can return either self or another value (e.g. -> (self | nil)).

Here's the concrete example: https://github.com/DataDog/dd-trace-rb/blob/5361e8e9a7092ca081ff0d5126daf8950bb2c792/lib/datadog/tracing/span_operation.rb#L202-L223

      def stop(stop_time = nil)
        return if stopped?

        now = Core::Utils::Time.now.utc

        start(stop_time || now) unless started?

        @end_time = stop_time || now
        @duration_end = stop_time.nil? ? duration_marker : nil

        events.after_stop.publish(self)

        self
      end

Currently this fails with:

Cannot allow method body have type `self` because declared as type `(self | nil)`
Diagnostic ID: Ruby::MethodBodyTypeMismatch

[ParadoxV5]

Could be similar:

def example(x) = x

class Object
  def example: [X < top] (X) -> X?
end

# Type checking files:

.F

ruby.rb:1:4: [error] Cannot allow method body have type `X` because declared as type `(X | nil)`
│   X <: (X | nil)
│     top <: (X | nil)
│       top <: X
│
│ Diagnostic ID: Ruby::MethodBodyTypeMismatch
│
└ def example(x) = x
      ~~~~~~~