Make diagnostic for @MetaSerializable on nested annotation

K2 plugin API has a limitation that meta annotations from plugin predicates
can't be used on nested annotations because their resolve process includes supertypes
resolve, that can be affected by the plugin itself. Therefore, @MetaSerializable
can't be applied to nested annotation classes in K2, which is reflected by
this diagnostic.

For old FE, diagnostic is lowered to WARNING with deprecation message.

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

@@ -0,0 +1,21 @@
+// WITH_STDLIB
+// SKIP_TXT
+
+import kotlinx.serialization.*
+
+@MetaSerializable
+annotation class TopLevel
+
+class MetaSerializableNestedTest {
+    <!META_SERIALIZABLE_NOT_APPLICABLE!>@MetaSerializable<!>
+    @Target(AnnotationTarget.PROPERTY, AnnotationTarget.CLASS)
+    annotation class JsonComment(val comment: String)
+
+    object Nested2 {
+        <!META_SERIALIZABLE_NOT_APPLICABLE!>@MetaSerializable<!>
+        annotation class Nested3
+    }
+
+    @JsonComment("class_comment")
+    data class IntDataCommented(val i: Int)
+}