IR: make DescriptorMetadataSource a subtype of MetadataSource

FirMetadataSource is a metadata source too.

plugins/android-extensions/android-extensions-compiler/src/org/jetbrains/kotlin/android/parcel/ir/ParcelableIrTransformer.kt

@@ -123,7 +123,7 @@ class ParcelableIrTransformer(private val context: IrPluginContext, private val
                     irExprBody(irInt(flags))
                 }
 
-                metadata = MetadataSource.Function(
+                metadata = DescriptorMetadataSource.Function(
                     declaration.descriptor.findFunction(ParcelableSyntheticComponent.ComponentKind.DESCRIBE_CONTENTS)!!
                 )
             }
@@ -178,7 +178,7 @@ class ParcelableIrTransformer(private val context: IrPluginContext, private val
                     }
                 }
 
-                metadata = MetadataSource.Function(
+                metadata = DescriptorMetadataSource.Function(
                     declaration.descriptor.findFunction(ParcelableSyntheticComponent.ComponentKind.WRITE_TO_PARCEL)!!
                 )
             }