Serialization: Fix serialization of PackElementType

Author: slavapestov

lib/Serialization/Serialization.cpp

@@ -5643,6 +5643,7 @@ void Serializer::writeAllDeclsAndTypes() {
   registerDeclTypeAbbr<OptionalTypeLayout>();
   registerDeclTypeAbbr<DynamicSelfTypeLayout>();
   registerDeclTypeAbbr<PackExpansionTypeLayout>();
+  registerDeclTypeAbbr<PackElementTypeLayout>();
   registerDeclTypeAbbr<PackTypeLayout>();
   registerDeclTypeAbbr<SILPackTypeLayout>();