clear savestack on (?{...}) failure and backtrack

clear savestack on (?{...}) failure and backtrack

RT #126697

In a regex, after executing a (?{...}) code block, if we fail and
backtrack over the codeblock, we're supposed to unwind the savestack, so
that for any example any local()s within the code block are undone.

It turns out that a backtracking state isn't pushed for (?{...}), only
for postponed evals ( i.e.  (??{...})). This means that it relies on one
of the earlier backtracking states to clear the savestack on its behalf.
This can't always be relied upon, and the ticket above contains code where
this falls down; in particular:

    'ABC' =~ m{
        \A
        (?:
            (?: AB | A | BC )
            (?{
                local $count = $count + 1;
                print "! count=$count; ; pos=${\pos}\n";
            })
        )*
        \z
    }x

Here we end up relying on TRIE_next to do the cleaning up, but TRIE_next
doesn't, since there's nothing it would be responsible for that needs
cleaning up.

The solution to this is to push a backtrack state for every (?{...}) as
well as every (??{...}). The sole job of that state is to do a
LEAVE_SCOPE(ST.lastcp).

The existing backtrack state EVAL_AB has been renamed EVAL_postponed_AB
to make it clear it's only used on postponed /(??{A})B/ regexes, and a new
state has been added, EVAL_B, which is only called when backtracking after
failing something in the B in /(?{...})B/.