Fixes bug #15654 bizarre constant mangling in 5.8.0

Fixes bug #15654 bizarre constant mangling in 5.8.0
What happened was that a constant was freed, the pad released but
the pad slot still held the SV, when pad slot was reallocated
to be a target for a stringify, it did a sv_setpv on the target
and the original SV was wiped out. When this SV was later on
to new places using the constant, they got the wrong value.
By replacing pad_free with pad_swipe for these cases, we
won't have such a problem. (pad_swipe also removes the
pointer to the original SV).

p4raw-id: //depot/perl@18820