Before this commit, `clone` gave different results depending on whether the original object
had an attached singleton class or not.

Consider the following setup:
```
class Foo; end
Foo.singleton_class.define_method(:foo) {}

obj = Foo.new

obj.singleton_class if $call_singleton

clone = obj.clone
```

When `$call_singleton = false`, neither `obj.singleton_class.singleton_class` nor
`clone.singleton_class.singleton_class` own any methods.

However, when `$call_singleton = true`, `clone.singleton_class.singleton_class` would own a copy of
`foo` from `Foo.singleton_class`, even though `obj.singleton_class.singleton_class` does not.

The latter case is unexpected and results in a visibly different clone, depending on if the original object
had an attached class or not.

Co-authored-by: Ufuk Kayserilioglu <ufuk.kayserilioglu@shopify.com>