Fix GH-20329: Remove CE cache from non-interned file cache strings

[arnaud-lb]

Possible fix for GH-20329.

• In SERIALIZE_STR(), ensure that we update the right copy of the string. ptr is an interior pointer to script->mem, which is SHM or a temporary serialization of the script. buf is the copy of script->mem that will be written to the file cache.
• Remove the IS_STR_CLASS_NAME_MAP_PTR flag

NB: I'm wondering if the if (script->corrupted) branch is useful, as we update flags in the same way in UNSERIALIZE_STR().