Run configuration "IDEA" doesn't help — it depends on "Build Kotlin Artifact", but Kotlin Plugin Artifact doesn't use actual compiler output — it uses dist/* libs and classes instead of out/production. So, I must explictly run ant build — it is painful.
Currently, there's no other way of doing this if you are making changes to the compiler. On the other hand, if you only work in the plugin part, there's no need to run the build.xml script every time.
The point of doing it this way is to make sure we debug exactly the same configuration as the users have. Before introducing this there were numerous failures caused by the difference between running the plugin in a debug session and installing it properly.