Improve definition handling for guessed receiver types

[st0012]

It's not uncommon for Ruby programmers to name variables after a class's superclass. But with guessed receiver types, such usages would actually reduce the number of definitions returned.

For example, given the following code:

class Animal
end

class Cat < Animal
  def meow
    "Meow"
  end
end

animal = Cat.new
animal.meow # meow can't be found with experimented features enabled

This commit improves the definition handling for guessed receiver types by treating them as unknown when no methods are found.