More accurate computation of compile job output paths

[owenv]

With this change, we're down to 6 interpreter and 3 stdlib lit tests still failing. I'm not all that happy with this solution, it feels kind of fragile. I haven't come up with anything better though so I thought I'd put this up in case anyone had a better solution in mind.

[owenv]

There's probably still room for improvement here, but with a few updates we now can pass all but one stdlib test

[cltnschlosser]

I don't fully understand the OutputFileMap, but could we add this to there instead of injecting a different path? or would that not work?

[owenv]

@cltnschlosser it might work, I need to look into it some more. There are some tricky cases where the inputs that produce the specified outputs might be temporary files.

[owenv]

Marking this as WIP until I have time to look over it again and come up with a better design

[owenv]

I decided to move the output path computation down into compile job planning. This now aims to reproduce the logic from the cpp getOutputFilename, excluding anything that isn't applicable to compile jobs. This makes it significantly easier to understand, but could lead to duplication throughout the other jobs in the future. Using a top-of-tree swift checkout, it passes the entire stdlib suite, and all but 5 interpreter tests. I'm not sure my isTopLevel check is 100% correct though.

[DougGregor]

Nice cleanup, thank you!

[DougGregor]

@swift-ci please test