KT-34194: Pass error/NonExistentClass for incremental KAPT run

When running KAPT incrementally, make sure to pass error/NonExistentClass.java
when there is at least one source file to process. This is to make sure
APs that must resolve all types are able to do so.

Test: updated KaptIncrementalWithAggregating/WithIsolating

plugins/kapt3/kapt3-base/src/org/jetbrains/kotlin/kapt3/base/KaptOptions.kt

@@ -148,7 +148,21 @@ fun KaptOptions.collectJavaSourceFiles(sourcesToReprocess: SourcesToReprocess =
 
     return when (sourcesToReprocess) {
         is SourcesToReprocess.FullRebuild -> allSources()
-        is SourcesToReprocess.Incremental -> sourcesToReprocess.toReprocess.filter { it.exists() }
+        is SourcesToReprocess.Incremental -> {
+            val toReprocess = sourcesToReprocess.toReprocess.filter { it.exists() }
+            if (toReprocess.isNotEmpty()) {
+                // Make sure to add error/NonExistentClass.java when there are sources to re-process, as
+                // this class is never reported as changed. See https://youtrack.jetbrains.com/issue/KT-34194 for details.
+                val nonExistentClass = stubsOutputDir.resolve("error/NonExistentClass.java")
+                if (nonExistentClass.exists()) {
+                    toReprocess + nonExistentClass
+                } else {
+                    toReprocess
+                }
+            } else {
+                emptyList()
+            }
+        }
     }
 }