[perl #114018] Let eval close over stale vars in active sub

[perl #114018] Let eval close over stale vars in active sub

See also commit cae5dbbe30.

These two lines should never produce different values:

    print $x, "\n";
    print eval '$x', "\n";

But they were producing different values if $x happened to have the
tale flag set.  Even if my in false conditional is not supported (this
was the cause of the bug report), it should still work; and it is
not the only way to get a stale lexical in an active sub (just the
easiest way).

As long as the sub containing the eval is active, the eval should be
able to see the same variables, stale or not.

However, this does get a bit tricky in cases like this, which legiti-
mately warn (from t/lib/warnings/pad):

{
    my $x = 1;
    $y = \$x; # force abandonment rather than clear-in-place at scope exit
    sub f2 { eval '$x' }
}
f2();

In this case the f2 sub does not explicitly close over the $x, so by
the time the eval is reached the ‘right’ $x is gone.

It is only in those cases where the sub containing the eval has
the stale variable in its own pad that we can safely ignore the
stale flag.