Revert the parts of #3926 that outlawed character ranges

Revert the parts of #3926 that outlawed character ranges
that have character classes such as \w as either endpoint.
This change re-establishes the old behavior which meant that
such ranges weren't really ranges, the "-" was literal.
Moreover, this change also fixes the old behavior to be
more consistent: [\w-.] and [\s-\w] worked, but [.-\w] didn't.
Now they all do work as described above.  The #3926 outlawed
all of those.

p4raw-id: //depot/cfgperl@4355