op.c: re-enable coderef-in-stash optimization

When seeing 'sub foo { ... }', perl does not need to allocate a full
typeglob (with the subroutine being stored in the CODE slot). Instead,
it can just store a coderef directly in the stash.

This optimization was first announced in perl5220delta:

> - Subroutines in packages no longer need to be stored in typeglobs:
>   declaring a subroutine will now put a simple sub reference directly
>   in the stash if possible, saving memory. The typeglob still
>   notionally exists, so accessing it will cause the stash entry to be
>   upgraded to a typeglob (i.e. this is just an internal implementation
>   detail).  This optimization does not currently apply to XSUBs or
>   exported subroutines, and method calls will undo it, since they
>   cache things in typeglobs.  [GH #13392]

However, due to a bug this optimization didn't actually work except for
package main (GH #15671). The issue was fixed in v5.27.5, but the fix
was backed out again in v5.27.9 because of CPAN breakage.

This patch re-enables the optimization because I want to see what the
current state of CPAN breakage is.

Author: mauke

op.c

@@ -10819,12 +10819,9 @@ Perl_newATTRSUB_x(pTHX_ I32 floor, OP *o, OP *proto, OP *attrs,
            Also, we may be called from load_module at run time, so
            PL_curstash (which sets CvSTASH) may not point to the stash the
            sub is stored in.  */
-        /* XXX This optimization is currently disabled for packages other
-               than main, since there was too much CPAN breakage.  */
         const I32 flags =
            ec ? GV_NOADD_NOINIT
               :   (IN_PERL_RUNTIME && PL_curstash != CopSTASH(PL_curcop))
-               || PL_curstash != PL_defstash
                || memchr(name, ':', namlen) || memchr(name, '\'', namlen)
                     ? gv_fetch_flags
                     : GV_ADDMULTI | GV_NOINIT | GV_NOTQUAL;

t/op/sub.t

@@ -398,10 +398,7 @@ is ref($main::{rt_129916}), 'CODE', 'simple sub stored as CV in stash (main::)';
     package RT129916;
     sub foo { 42 }
 }
-{
-    local $::TODO = "disabled for now";
-    is ref($RT129916::{foo}), 'CODE', 'simple sub stored as CV in stash (non-main::)';
-}
+is ref($RT129916::{foo}), 'CODE', 'simple sub stored as CV in stash (non-main::)';
 
 # Calling xsub via ampersand syntax when @_ has holes
 SKIP: {