[Compiler] Mark all entrypoints to compiler API as experimental

buildSrc/src/main/kotlin/tasks.kt

@@ -330,3 +330,11 @@ fun Project.confugureFirPluginAnnotationsDependency(testTask: TaskProvider<Test>
         }
     }
 }
+
+fun Project.optInToExperimentalCompilerApi() {
+    tasks.withType<org.jetbrains.kotlin.gradle.dsl.KotlinCompile<*>>().configureEach {
+        kotlinOptions {
+            freeCompilerArgs += "-opt-in=org.jetbrains.kotlin.compiler.plugin.ExperimentalCompilerApi"
+        }
+    }
+}