Rename platform to header.

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/NotFoundClasses.kt

@@ -100,7 +100,7 @@ class NotFoundClasses(private val storageManager: StorageManager, private val mo
 
         override fun isCompanionObject() = false
         override fun isData() = false
-        override fun isPlatform() = false
+        override fun isHeader() = false
         override fun isImpl() = false
         override fun isExternal() = false
         override val annotations: Annotations get() = Annotations.EMPTY

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/descriptors/DeserializedClassDescriptor.kt

@@ -95,7 +95,7 @@ class DeserializedClassDescriptor(
 
     override fun isData() = Flags.IS_DATA.get(classProto.flags)
 
-    override fun isPlatform() = false
+    override fun isHeader() = false
 
     override fun isImpl() = false