[Bug 281478] devel/libxsd-frontend: fix build with libc++ 19

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281478

            Bug ID: 281478
           Summary: devel/libxsd-frontend: fix build with libc++ 19
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Individual Port(s)
          Assignee: kde at FreeBSD.org
          Reporter: dim at FreeBSD.org
             Flags: maintainer-feedback?(kde at FreeBSD.org)
          Assignee: kde at FreeBSD.org

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes www/qt5-webengine to fail to compile with clang 19 and
libc++ 19, resulting in errors similar to:

  /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined
template 'std::char_traits<XSDFrontend::Bits::None>'
    820 |   static_assert(is_same<_CharT, typename
traits_type::char_type>::value,
        |                                          ^
 
/wrkdirs/usr/ports/devel/libxsd-frontend/work/libxsd-frontend-2.1.0/libxsd-frontend/xml.hxx:100:23:
note: in instantiation of template class
'std::basic_string<XSDFrontend::Bits::None>' requested here
    100 |       NarrowString r (s);
        |                       ^
  /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
     23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
        |                             ^

The `Bits::None` construct used by libxsd-frontend is meant to disable
unwanted conversions, but it no longer works if `std::char_traits>` is
undefined for this type.

Fix it by removing the `Bits` namespace and only enabling the converting
constructors if the first type of `StringTemplate` is `wchar_t`.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

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