[perl #86136] Downgrade sort {my $a} to a warning

[perl #86136] Downgrade sort {my $a} to a warning

The code in toke.c for detecting lexical $a or $b used in a comparison
in a sort block was simply horrible.  If the last-used named list or
unary op (PL_last_lop_op) was sort, then it would scan for <=> or cmp
anywhere on the current line of code.  That meant that, although this
would die:

    my $a; sort { $a <=> $b } ()

This would do the wrong thing without complaint:

    my $a; sort { print; $a <=> $b } ()

And this would die, completely gratuitously:

    my $a; sort @t; $a + $cmp;

Since perl is only guessing that lexical $a or $b *might* have
been used accidentally, this should be a warning, and certainly
not an error.

Also, scanning the source code like that for <=> (even inside a
string!) can never work.  One would have to parse it and examine the
resulting op tree.

In fact, since we *are* parsing it anyway, we *can* examine
the op tree.

So that’s exactly what this commit does.  Based on the existing behav-
iour, but with far fewer false positives, it checks for a cmp or <=>
op as the last statement of a sort block and warns about any operand
that is a lexical $a or $b.