Strings loaded from the file cache can not have a CE cache, because their cache
slot is invalid. Remove the IS_STR_CLASS_NAME_MAP_PTR flag from these strings.
We can also avoid updating the str flags in SERIALIZE_STR(), since the same
updates must also be done in UNSERIALIZE_STR().
This was already done for interned strings, but not for non-interned ones.
Fixes GH-20329
Closes GH-20337
b062410d32