Integrate:

Integrate:
[ 24921]
weak references aren't UVs, do don't show this in the dump flags.

[ 24924]
Avoid having NULL entries in the weakref backreference array, and
make S_sv_add_backref O(1) (instead of O(n))
p4raw-link: @24924 on //depot/perl: 6a76db8b55b6281740baab9e0ef221938c776ccd
p4raw-link: @24921 on //depot/perl: 69c678eb3bbbed7a7896fd9a2e52b0bbb93d4c3e

p4raw-id: //depot/maint-5.8/perl@25415
p4raw-integrated: from //depot/perl@24924 'merge in' sv.c (@24918..)
p4raw-integrated: from //depot/perl@24921 'merge in' dump.c (@24798..)