Fix lost nullability in C-interop type aliases

^KT-43427

kotlin-native/Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StubIrMetadataEmitter.kt

@@ -326,6 +326,11 @@ private class MappingExtensions(
                 Flag.Type.IS_NULLABLE.takeIf { nullable }
         )
 
+    val AbbreviatedType.expandedTypeFlags: Flags
+        get() = flagsOfNotNull(
+                Flag.Type.IS_NULLABLE.takeIf { isEffectivelyNullable() }
+        )
+
     val TypealiasStub.flags: Flags
         get() = flagsOfNotNull(
                 Flag.IS_PUBLIC
@@ -357,6 +362,13 @@ private class MappingExtensions(
                 Flag.HAS_ANNOTATIONS.takeIf { annotations.isNotEmpty() }
         ) or visibility.flags
 
+    private tailrec fun StubType.isEffectivelyNullable(): Boolean =
+            when {
+                nullable -> true
+                this !is AbbreviatedType -> false
+                else -> underlyingType.isEffectivelyNullable()
+            }
+
     fun AnnotationStub.map(): KmAnnotation {
         fun Pair<String, String>.asAnnotationArgument() =
                 (first to KmAnnotationArgument.StringValue(second)).takeIf { second.isNotEmpty() }
@@ -467,8 +479,7 @@ private class MappingExtensions(
                 km.arguments += typeArguments
             }
             if (shouldExpandTypeAliases) {
-                // Abbreviated and expanded types have the same nullability.
-                KmType(flags).also { km ->
+                KmType(expandedTypeFlags).also { km ->
                     km.abbreviatedType = abbreviatedType
                     val kmUnderlyingType = underlyingType.map(true)
                     km.arguments += kmUnderlyingType.arguments