Serialization: Fix deserialization of generic typealiases

The recovery logic was erronously kicking in, because it was comparing
the substituted underlying type with the declaration's underlying type.

For a generic typealias, these never equal, so instead, serialize the
unsubstituted type, and substitute it in deserialization.

lib/Serialization/Serialization.cpp

@@ -3625,13 +3625,14 @@ void Serializer::writeType(Type ty) {
   case TypeKind::NameAlias: {
     auto alias = cast<NameAliasType>(ty.getPointer());
     const TypeAliasDecl *typeAlias = alias->getDecl();
+    auto underlyingType = typeAlias->getUnderlyingTypeLoc().getType();
 
     unsigned abbrCode = DeclTypeAbbrCodes[NameAliasTypeLayout::Code];
     NameAliasTypeLayout::emitRecord(
                            Out, ScratchRecord, abbrCode,
                            addDeclRef(typeAlias, /*allowTypeAliasXRef*/true),
                            addTypeRef(alias->getParent()),
-                           addTypeRef(alias->getSinglyDesugaredType()),
+                           addTypeRef(underlyingType),
                            addSubstitutionMapRef(alias->getSubstitutionMap()));
     break;
   }