gdb/symtab: Allow changing of added cooked_index entries

Make cooked_index_storage::add and cooked_index_entry::add return a
"cooked_index_entry *" instead of a "const cooked_index_entry *".

Tested on x86_64-linux and ppc64le-linux.
Tested-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>