Don't bother doing POSIX charclass parsing if it

Don't bother doing POSIX charclass parsing if it
possibly cannot be so.  Prepares way for charclass
syntax like [[abc]||[def]] (or just [[abc][def]])
for union, [[\w]&&[$a]] for intersection,
and [[a-z]&&[^def]] for subtraction.

Currently /[[a]/ (or /[a[]/) parses as a character
class containing two characters, "[" and "a",
this may have to be broken for the syntax described
above, otherwise we would have to scan the whole pattern
to find out whether the square brackets match pairwise.
Luckily, the special case of "[" doesn't seem to be
documented (as opposed to "]" and "-"), so we may have
better story for breaking it...  One can always use \[
if one wants a literal "[", so there.

p4raw-id: //depot/perl@12835