Give regex_traits::__regex_word a proper definition.

[ajwong]

Summary

Give regex_traits::regex_word a proper definition.
Without this, libcxx can fail to link because regex_word will not have
storage allocated for it.

[ajwong]

Move constant specifier back to declaration.

include/regex

private:
     static int __value(unsigned char __ch, int __radix);
     _LIBCPP_INLINE_VISIBILITY
     int __value(char __ch, int __radix) const
         {return __value(static_cast<unsigned char>(__ch), __radix);}
     int __value(wchar_t __ch, int __radix) const;
 };
 
 template <class _CharT>
+const typename regex_traits<_CharT>::char_class_type
+regex_traits<_CharT>::__regex_word;
+
+template <class _CharT>
 regex_traits<_CharT>::regex_traits()
 {
     __init();
 }
 
 template <class _CharT>
 typename regex_traits<_CharT>::char_type
 regex_traits<_CharT>::translate_nocase(char_type __c) const