eval: ensure debugging saved lines have an IV part

perldebguts documents that the lines stored in @{"_<$filename"}
arrays have a numeric value in addition to the text of the source,
ensure that is true for evals.

Non-zero IV values indicate the lines are breakable (they represent
the address of the COP for that line)

Fixes #23151

Author: tonycoz

MANIFEST

@@ -6307,6 +6307,7 @@ t/op/cproto.t				Check builtin prototypes
 t/op/crypt.t				See if crypt works
 t/op/current_sub.t			__SUB__ tests
 t/op/dbm.t				See if dbmopen/dbmclose work
+t/op/debug.t				Test mechanisms used by the debugger
 t/op/decl-refs.t			See if my \$foo works
 t/op/defer.t				See if defer blocks work
 t/op/defined.t				See if defined() edge cases work

pp_ctl.c

@@ -3744,6 +3744,9 @@ S_save_lines(pTHX_ AV *array, SV *sv)
             t = send;
 
         sv_setpvn_fresh(tmpstr, s, t - s);
+        /* not breakable until we compile a COP for it */
+        SvIV_set(tmpstr, 0);
+        SvIOK_on(tmpstr);
         av_store(array, line++, tmpstr);
         s = t;
     }

t/op/debug.t

@@ -0,0 +1,64 @@
+#!./perl
+
+# intended for testing language mechanisms that debuggers use not for
+# testing the debugger itself
+
+BEGIN {
+    chdir 't' if -d 't';
+    require "./test.pl";
+    set_up_inc( qw(. ../lib) );
+}
+
+use strict;
+use warnings;
+
+SKIP:
+{
+    skip_if_miniperl("need XS", 1);
+    # github 23151
+    # trivial debugger
+    local $ENV{PERL5DB} = 'sub DB::DB {}';
+    # eval code trimmed from code generated by Sub::Quote
+    fresh_perl_is(<<'CODE', <<'EXPECT',
+use B qw(SVf_IOK);
+
+sub _do_eval {
+  eval $_[0] or die $!;
+}
+
+_do_eval(<<'EVAL');
+{
+  sub table {
+  }
+}
+1;
+EVAL
+
+# look for lines that don't have an IV set
+my ($f) = grep /\(eval/, keys %::;
+my $x = $::{$f};
+my $lineno = 0;
+for my $l (@$x) {
+  if ($l) {
+    my $b = B::svref_2object(\$l);
+    if (!($b->FLAGS & SVf_IOK)) {
+      print "No IV for $f line $lineno: $l\n";
+      last
+    }
+  }
+  ++$lineno;
+}
+
+print "Done\n";
+CODE
+Done
+EXPECT
+                  {
+                      switches => [ '-d' ],
+                      stderr => 1,
+                  },
+                  "saved lines all have an IV"
+                  );
+}
+
+done_testing();