[FIR] Fix warnings in FIR modules and enable -Werror in them

compiler/fir/tree/build.gradle.kts

@@ -50,6 +50,12 @@ val compileKotlin by tasks
 
 compileKotlin.dependsOn(generateTree)
 
+tasks.withType<org.jetbrains.kotlin.gradle.dsl.KotlinCompile<*>> {
+    kotlinOptions {
+        freeCompilerArgs += "-Xinline-classes"
+    }
+}
+
 if (kotlinBuildProperties.isInJpsBuildIdeaSync) {
     apply(plugin = "idea")
     idea {