[Bug 281499] devel/folly: fix build with clang 19

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281499

            Bug ID: 281499
           Summary: devel/folly: fix build with clang 19
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Individual Port(s)
          Assignee: yuri@freebsd.org
          Reporter: dim@FreeBSD.org
          Assignee: yuri@freebsd.org
             Flags: maintainer-feedback?(yuri@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 devel/folly to fail to compile with clang 19 and libc++ 19,
resulting in errors similar to:

  /usr/include/c++/v1/string_view:300:42: error: implicit instantiation of
undefined template 'std::char_traits<unsigned char>'
    300 |   static_assert(is_same<_CharT, typename
traits_type::char_type>::value,
        |                                          ^
 
/wrkdirs/share/dim/ports/devel/folly/work/folly-2024.09.09.00/folly/io/Cursor.h:686:35:
note: in instantiation of template class 'std::basic_string_view<unsigned
char>' requested here
    686 |   std::basic_string_view<uint8_t> peekView() {
        |                                   ^
  /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
     23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
        |                             ^

Since the peekView() function is supposed to return a no longer existing
type, conditionalize it for libc++ versions before 19. As far as I can
see, there are no consumers of this function, except for some of the
tests.

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

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