The AudioKit package was seeing an assertion failure with Swift 5.1 that has been fixed in Swift 5.2. This forum comment: https://forums.swift.org/t/llvm-crashes-in-xcode-11-2-with-build-libraries-for-distribution/30515/4 contains the main function that triggered this issue. It would be good to have a regression test for this in the test suite, even though the problem has been fixed since.
The way to make sure that this test does the thing we expect it to do, is to pick a couple of tags, one swift-5.1-DEVELOPMENT-SNAPSHOT-20yy-mm-dd ("known bad") and similarly for 5.2 ("known good") and manually test that one triggers an assertion failure and another one doesn't. It might take a little bit of trial and error to find two tags that do the expected thing, so it make sense to use two different build directories (using build-script's --build-subdir flag, say) so you're not constantly overwriting the build when switching between tags while working on the PR/reducing the test case.
Please feel free to tag me (varungandhi-apple) for review on GitHub and/or leave a comment here if you need guidance.