Allow union types to be unbound

[HertzDevil]

Fixes #9173. In

class Object
  def foo
    self
  end
end

abstract class Parent
end

class Child(T) < Parent
  @buffer = uninitialized T

  def bar
    @buffer.foo # Error: undefined method 'foo' for U (compile-time type is (Char | U))
  end
end

class Foo(U)
  @x = Child(U | Char).new
end

Child(Int32).new.as(Parent).bar

When Parent#bar is called the compiler runs type inference on the implementations in all concrete subtypes of it. This incorrectly included Child(U | Char) because it and U | Char were not considered unbound types. This PR fixes that. (In the original issue this is triggered through Array(T | U)#inspect of an instance variable inside Iterator::InGroupsOf(I, T, N, U).)