[FIR] Store generated declarations in the session component

Also extract generated declarations for classes from declaration site
  session, not use site

^KT-57821 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/K2KotlinxSerializationIT.kt

@@ -69,4 +69,12 @@ class K2KotlinxSerializationIT : KGPBaseTest() {
             build(":compileTestKotlinJs")
         }
     }
+
+    @DisplayName("Compile MPP project to JS kotlinx.serialization and K2")
+    @GradleTest
+    fun `test kotlinx serialization mpp to JS`(gradleVersion: GradleVersion) {
+        project("kotlinxSerializationMppK2", gradleVersion) {
+            build(":compileKotlinJs")
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationMppK2/src/jsMain/kotlin/JsMain.kt

@@ -0,0 +1,3 @@
+fun someSerializerCall() {
+    ValidateViolation.serializer()
+}