devel/llvm12: fix build with libc++ 19

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/llvm12 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<unsigned char>'
      820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
          |                                          ^
    /wrkdirs/usr/ports/devel/llvm12/work-default/llvm-project-12.0.1.src/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp:578:34: note: in instantiation of template class 'std::basic_string<unsigned char>' requested here
      638 |       std::basic_string<uint8_t> zeros(reg_info.byte_size, '\0');
          |                                  ^
    /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
       23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
          |                             ^

Upstream llvm has fixed this in commit 68744ffbdd7d [2], so add this as
a backported patch file.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] https://github.com/llvm/llvm-project/commit/68744ffbdd7d

PR:		280579
Approved by:	brooks (maintainer)
MFH:		2024Q3

(cherry picked from commit 0626947c)