[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()
+    }
+}

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

@@ -10,8 +10,6 @@ import java.util.*
 
 @Serializer(forClass = Date::class)
 object DateSerializer : KSerializer<Date> {
-    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("java.util.Date", PrimitiveKind.STRING)
-
     override fun serialize(encoder: Encoder, value: Date) {
         TODO()
     }

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.fir.txt

@@ -181,7 +181,7 @@ FILE: serializerViaCompanion.kt
         public final val i: R|kotlin/Int| = R|<local>/i|
             public get(): R|kotlin/Int|
 
-        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/FullyOverridden|)) public final companion object Companion : R|kotlinx/serialization/KSerializer<com/example/FullyOverridden>| {
+        public final companion object Companion : R|kotlinx/serialization/KSerializer<com/example/FullyOverridden>| {
             private constructor(): R|com/example/FullyOverridden.Companion| {
                 super<R|kotlin/Any|>()
             }

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt

@@ -67,8 +67,8 @@ data class PartiallyWithoutType(val i: Int) {
 
 @Serializable(FullyOverridden.Companion::class)
 data class FullyOverridden(val i: Int) {
-    @Serializer(forClass = FullyOverridden::class)
     companion object : KSerializer<FullyOverridden> {
+
         override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("FullyOverridden", PrimitiveKind.STRING)
 
         override fun serialize(encoder: Encoder, value: FullyOverridden) {