Nothing's really changed with that recently. Still using a fork (though less divergent than it has been?), but it also builds fine with upstream.

Are there plans to merge remaining differences to upstream and switch to it?

We generally send our patches upstream but they can take some time to land. This will just always be the case. We test that everything builds with stock LLVM, you’ll probably just get some test failures from the unpatched bugs.

So the fork is mostly needed for convenient fast iteration?

Yup.