Ensure singleton class is created when we find an extend

[vinistock]

I noticed that in the following snippet:

class Foo
  extend Bar
end

It was not possible to ask any ancestor information about Foo::<Foo>. We need to ensure that the singleton is created if an extend happens. This PR ensures that.

[vinistock]

• Ensure unresolved parent classes appear as partial ancestors #734
• Ensure singleton class is created when we find an extend #730 👈 (View in Graphite)
• Fix top level constant resolution #729 : 1 other dependent PR (#731 )
• Fix default parent classes #728
• main

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[vinistock]

Merge activity

• Apr 15, 2:55 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 15, 3:03 PM UTC: Graphite rebased this pull request as part of a merge.
• Apr 15, 3:08 PM UTC: @vinistock merged this pull request with Graphite.