Kapt: Attach generated Kotlin sources in 'compilation' mode (KT-32535)

plugins/kapt3/kapt3-cli/testData/integration/kotlinFileGeneration/ap/META-INF/services/javax.annotation.processing.Processor

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+apt.SampleApt

plugins/kapt3/kapt3-cli/testData/integration/kotlinFileGeneration/ap/Processor.kt

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+package apt
+
+import java.io.File
+import javax.annotation.processing.*
+import javax.lang.model.SourceVersion
+import javax.lang.model.element.TypeElement
+import javax.tools.Diagnostic.Kind.*
+
+annotation class Anno
+
+class SampleApt : AbstractProcessor() {
+    private companion object {
+        const val KAPT_KOTLIN_GENERATED_OPTION_NAME = "kapt.kotlin.generated"
+    }
+
+    override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
+        val kaptKotlinGeneratedDir = processingEnv.options[KAPT_KOTLIN_GENERATED_OPTION_NAME] ?: run {
+            processingEnv.messager.printMessage(ERROR, "Can't find the target directory for generated Kotlin files.")
+            return false
+        }
+
+        val baseDir = File(kaptKotlinGeneratedDir, "generated")
+        baseDir.mkdirs()
+
+        for (element in roundEnv.getElementsAnnotatedWith(Anno::class.java)) {
+            val generatedSimpleName = element.simpleName.toString().capitalize()
+            val file = File(baseDir, "$generatedSimpleName.kt")
+            file.writeText("package generated\nclass $generatedSimpleName")
+        }
+
+        return true
+    }
+
+    override fun getSupportedOptions() = emptySet<String>()
+    override fun getSupportedSourceVersion() = SourceVersion.RELEASE_8
+    override fun getSupportedAnnotationTypes() = setOf("apt.Anno")
+}