devel/folly: fix build with clang 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/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
PR: 281499
Approved by: yuri (maintainer)
MFH: 2024Q3
(cherry picked from commit b3459442f762df2cf1e81c582c78da1481c834f6)