I already submitted a major rework of the tests before seeing your comments. This open-source phabricator does not keep a history of diff revisions, so I can't see the original version on which you commented. Have a look at the latest and tell me if the right column still needs attention.
All devs do incremental builds. Of course incremental builds should work.
I went ahead and fixed this in https://github.com/llvm/llvm-project/commit/0389eff4047a74bb1ba6c0603c9002b5c73b203e
Going forward, please don't leave stuff broken overnight.