Perl_leave_scope - sv_backoff shouldn't do an unnecessay string copy

When a `my` SV goes out of scope, any OOK hack on its string buffer is
undone by `Perl_sv_backoff`. If the SV is `SvOK`, a copy of the buffer
contents will occur, but since the contents are defunct at this point,
the copy is unnecessary.

See #23967 as an example of where
this unnecessary copy had a noticeable effect on performance.

This commit adds some flag meddling either side of the `sv_backoff` call
to avoid the copy but otherwise preserve the SV's flags.

Author: richardleach

scope.c

@@ -1440,9 +1440,20 @@ Perl_leave_scope(pTHX_ I32 base)
 
                         if (SvTYPE(sv) == SVt_PVHV && HvHasAUX(sv))
                             Perl_hv_kill_backrefs(aTHX_ MUTABLE_HV(sv));
-                        else if(SvOOK(sv))
+                        else if(SvOOK(sv)) {
+                            /* sv_backoff will copy the buffer contents when SvOK(sv),
+                               even though this particular buffer is defunct and copying
+                               its contents is a waste of time. It might not (?) be
+                               desirable to remove any SvOK flags just yet, so we fake it.*/
+                            U32 savedflags = SvFLAGS(sv);
+                            savedflags &=~ SVf_OOK;
+                            SvFLAGS(sv) &=~ (SVf_OK);
+
                             sv_backoff(sv);
 
+                            SvFLAGS(sv) = savedflags;
+                        }
+
                         if (SvMAGICAL(sv)) {
                             /* note that backrefs (either in HvAUX or magic)
                              * must be removed before other magic */