Make COW-clobbering faster

Make COW-clobbering faster

There is this special SV_COW_DROP_PV flag that gets passed to
sv_force_normal_flags to signal that the SV is about to be clobbered,
so there is no point in allocating a new PV.  But this flag was not
actually being used, until the previous commit commandeered it for
globs (despite the name).

Now it actually does what it says.

Before and after:

$ time ./perl -e '$x = __PACKAGE__, undef $x for 1..8000000'

real	0m5.758s
user	0m5.740s
sys	0m0.008s
$ time ./perl -e '$x = __PACKAGE__, undef $x for 1..8000000'

real	0m3.290s
user	0m3.282s
sys	0m0.006s