[Serialization] Add DiagnosticsTests for FIR

plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.fir.kt

@@ -0,0 +1,17 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
+// WITH_STDLIB
+// FILE: test.kt
+import kotlinx.serialization.*
+
+class NonSerializable
+
+@Serializable
+class Basic(val foo: NonSerializable)
+
+@Serializable
+class Inside(val foo: List<NonSerializable>)
+
+@Serializable
+class WithImplicitType {
+    val foo = NonSerializable()
+}