This builds on D116521 to add this functionality to Polly without more copy-pasted code.
This is actually *not* working, CI reports. I haven't yet figured out why the same idiom that works everywhere else doesn't work here. Back to the drawing board.
Fix relatie path calculation issue in exports.
Instead reuse the same shared logic to get the path and do it that way.
This is the new bit, replacing the wold way of doing relative exports.
Someday, it would be nice to replace this the usual LLVMDistributionSupport methods, but that is a bigger change best left for later.