[kxSerialization] Added diagnostic on useless Serializer annotation

Resolves Kotlin/kotlinx.serialization#2182

Merge-request: KT-MR-8718
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt

@@ -4,8 +4,26 @@
 
 // FILE: test.kt
 import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
 
 class Foo(i: Int, val j: Int)
 
 <!EXTERNAL_CLASS_NOT_SERIALIZABLE!>@Serializer(forClass = Foo::class)<!>
 object ExternalSerializer
+
+<!EXTERNAL_SERIALIZER_USELESS!>@Serializer(forClass = Foo::class)<!>
+object UselessExternalSerializer : KSerializer<Foo> {
+    override val descriptor: SerialDescriptor
+        get() {
+            TODO()
+        }
+
+    override fun serialize(encoder: Encoder, value: Foo) {
+        TODO()
+    }
+
+    override fun deserialize(decoder: Decoder): Foo {
+        TODO()
+    }
+}