[FIR] Separate generated tree files from non-generated sources

compiler/fir/tree/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.ideaExt.idea
+
 /*
  * Copyright 2000-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
@@ -18,7 +20,10 @@ dependencies {
 }
 
 sourceSets {
-    "main" { projectDefault() }
+    "main" {
+        projectDefault()
+        this.java.srcDir("gen")
+    }
 }
 
 val generatorClasspath by configurations.creating
@@ -27,8 +32,10 @@ dependencies {
     generatorClasspath(project("tree-generator"))
 }
 
+val generationRoot = projectDir.resolve("gen")
+
 val generateTree by tasks.registering(NoDebugJavaExec::class) {
-    val generationRoot = "$projectDir/src/"
+
     val generatorRoot = "$projectDir/tree-generator/src/"
 
     val generatorConfigurationFiles = fileTree(generatorRoot) {
@@ -47,3 +54,10 @@ val generateTree by tasks.registering(NoDebugJavaExec::class) {
 val compileKotlin by tasks
 
 compileKotlin.dependsOn(generateTree)
+
+if (kotlinBuildProperties.isInJpsBuildIdeaSync) {
+    apply(plugin = "idea")
+    idea {
+        this.module.generatedSourceDirs.add(generationRoot)
+    }
+}