Refactor HscMain.finish

I found the old control flow a bit hard to follow; I rewrote it to first decide whether to desugar, and then use that choice when computing whether to simplify / what sort of interface file to write.

I hope eventually we will always write post-tc interface files, which will make the logic of this function even simpler, and continue the thrust of this refactor.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.