<regex>: Make negated character classes not match characters not included in the negated character class

[muellerj2]

Partially addresses #992 again. While there is no easy way to make negated character class escapes (in square-bracket character classes) match members of the class, we can at least make sure that such escapes don't match non-members.

Drive-by changes:

• Correct the limit check (although I think the correction has no practical consequences).
• Change _Builder::_Add_named_class and _Builder::_Add_elts to use the char_class_type of the traits class instead of _Regex_traits_base::char_class_type when adding a character class.
• Remove the default argument from _Builder::_Add_named_class.