Fix cords for MANUAL_VDB

Fix cords for MANUAL_VDB
(a cherry-pick of commit c72eeacc from 'release-7_6')

* cord/cordbscs.c (CORD_cat_char_star, CORD CORD_cat,
CORD_from_fn_inner, CORD_substr_closure): Call GC_END_STUBBORN_CHANGE.
* cord/cordxtra.c (refill_cache): Likewise.
* cord/tests/de.c (prune_map, add_map, add_hist, replace_line,
generic_init): Likewise.
* cord/cordbscs.c (CORD_cat_char_star, CORD_cat): Mark x and y as
reachable after GC_END_STUBBORN_CHANGE(result).
* cord/cordbscs.c (CORD_from_fn_inner): Mark client_data as reachable
after GC_END_STUBBORN_CHANGE(result).
* cord/cordbscs.c (CORD_substr_closure): Mark x as reachable after
GC_END_STUBBORN_CHANGE(sa).
* cord/tests/de.c (prune_map): Mark saved map->previous->previous as
reachable after GC_END_STUBBORN_CHANGE(map).
* cord/tests/de.c (add_map): Mark saved current_map as reachable after
GC_END_STUBBORN_CHANGE(new_map).
* cord/tests/de.c (replace_line): Mark s as reachable after
GC_END_STUBBORN_CHANGE(screen+i).