Avoid assert fail with s// $target = \3 /e

Avoid assert fail with s// $target = \3 /e

When the substitution target is assigned to in pp_substcont, it is
assumed that SvPV_free and SvPOK_only_UTF8 can be used on that target.
Only COW scalars are sent through sv_force_normal.

Changing the target in the replacement code can render those assump-
tions untrue:

$  ./perl -Ilib -e '$h = 3; $h =~ s/3/$h=\3;4/e'
Assertion failed: (!((targ)->sv_flags & 0x00000800) || !(*({ SV *const _svrv = ((SV *)({ void *_p = (targ); _p; })); (__builtin_expect(!(PL_valid_types_RV[((svtype)((_svrv)->sv_flags & 0xff)) & 0xf]), 0) ? __assert_rtn(__func__, "pp_ctl.c", 269, "PL_valid_types_RV[SvTYPE(_svrv) & SVt_MASK]") : (void)0); (__builtin_expect(!(!((((_svrv)->sv_flags & (0x00004000|0x00008000)) == 0x00008000) && (((svtype)((_svrv)->sv_flags & 0xff)) == SVt_PVGV || ((svtype)((_svrv)->sv_flags & 0xff)) == SVt_PVLV))), 0) ? __assert_rtn(__func__, "pp_ctl.c", 269, "!isGV_with_GP(_svrv)") : (void)0); (__builtin_expect(!(!(((svtype)((_svrv)->sv_flags & 0xff)) == SVt_PVIO && !(((XPVIO*) (_svrv)->sv_any)->xio_flags & 64))), 0) ? __assert_rtn(__func__, "pp_ctl.c", 269, "!(SvTYPE(_svrv) == SVt_PVIO && !(IoFLAGS(_svrv) & IOf_FAKE_DIRP))") : (void)0); &((_svrv)->sv_u.svu_rv); }))), function Perl_pp_substcont, file pp_ctl.c, line 269.
Abort trap: 6

Also, matching against a hash key and locking that key with Hash::Util
within the replacement code can cause the substitution to modify that
hash key without triggering ‘Modification of a read-only value’.  But
this only happens if it is not a copy-on-write scalar:

$ ./perl -Ilib -MHash::Util=lock_hash -le '$h{foo} = 3; $h{foo} =~ s/3/$h{foo} = 3; lock_hash %h; 4/e; print $h{foo}'
4

We need to do a regular SV_THINKFIRST_COW_DROP check here, just as we
do in sv_setsv with regular scalar assignment.

Also, we need to take into account real globs:

$ ./perl -Ilib -MHash::Util=lock_hash -le '$::{foo} =~ s//*{"foo"}; 4/e'
Assertion failed: (!isGV_with_GP(targ)), function Perl_pp_substcont, file pp_ctl.c, line 259.
Abort trap: 6