Swift incremental compilation does not work with large modules

[adincebic]

After spending quite some time on this, I determined that Swift incremental compilation via Bazel and rules_swift does not work. I am aware that there has been some progress made on this in the past and I believe that incremental compilation does work for smaller modules but not for ones that contain a couple of thousands source files.

I created a small reproducer project and provided more details on what I tried in the README file.
Find the repro here: https://github.com/adincebic/bazel-ios-swiftui-template/tree/ac/swift-incremental-compilation-not-working

[adincebic]

I don't actually think that it works for any kind of module but I didn't find a good way to verify that assumption.