[Parcelize] Ignore all sealed classes in Parcelize FIR generator

Sealed classes are inherently abstract and should not have Parcelable
functions added via the FIR declaration generator.

#KT-59112 Fixed

plugins/parcelize/parcelize-compiler/testData/diagnostics/modality.fir.kt

@@ -15,6 +15,9 @@ class Final(val foo: String) : Parcelable
 @Parcelize
 sealed class Sealed(val foo: String) : Parcelable {
     class X : Sealed("")
+    sealed class Inner : Sealed("") {
+        <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class Y<!> : Inner()
+    }
 }
 
 class Outer {

plugins/parcelize/parcelize-compiler/testData/diagnostics/modality.kt

@@ -15,6 +15,9 @@ class Final(val foo: String) : Parcelable
 @Parcelize
 sealed class Sealed(val foo: String) : Parcelable {
     class X : Sealed("")
+    sealed class Inner : Sealed("") {
+        <!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class Y<!> : Inner()
+    }
 }
 
 class Outer {