Fix splat fallback for union types

[pvcresin]

Summary

• Fix splat fallback for union types.
• Move the regression scenario into the regular array scenarios.

Details

[*x] now keeps receiver types without to_a separate from the normal to_a result, so each union member can be handled independently. Existing array and tuple inference remains unchanged.

Test

• bundle exec ruby -Itest test/scenario_test.rb
• bundle exec rake test

[mame]

Thanks! fallback feels like the name from the splat's side; I'd prefer something like unresolved_recv. It's trivial, so let me merge this as is and rename in a follow-up.