Ensure singleton class ancestors are linearized when created out of resolution loop

[vinistock]

In get_or_create_singleton_class, we were lazily creating singleton classes and enqueueing their ancestors for linearization. The problem is that we invoke this method for handling definitions too and that happens after the resolution loop finished, which means that the enqueue work was always ignored.

There are a few ways to solve this:

1. Drain the queue again after handling definitions
2. Move handling all definitions inside of the same unified loop
3. Create an eager/lazy mode for linearizing the ancestors of lazily created singleton classes, so that any calls to get_or_create_singleton_class inside of the resolution loop enqueue work and all others eagerly linearize

I believe 1 is not a good option as we'll need to loop through everything again, including unresolved references. Option 2 seems like the most elegant of all, but it might be a considerably large refactor, so I went with option 3 for now.

[vinistock]

• Index instance variable references #757
• Ensure singleton class ancestors are linearized when created out of resolution loop #735 👈 (View in Graphite)
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