Don’t let ?: folding affect truncate

Don’t let ?: folding affect truncate

truncate(${\1} ? foo : bar, 0) and truncate(1 ? foo : bar, 0) should
behave the same way, but were treated differently, due to the way ?:
is folded in the latter case.  Now that foldedness is recorded in the
op tree (cc2ebcd7902), we can use the OPpCONST_FOLDED flag to distin-
guish truncate(1 ? foo : bar, 0) from truncate(foo, 0).