fix Range under libc++ >= 19 where basic_string_view only allows char…

… types

Summary:
LLVM 19 removed the base templates for `std::char_traits` and only allows the following types to be used `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t`.

From the [release notes](https://github.com/llvm/llvm-project/blob/release/19.x/libcxx/docs/ReleaseNotes/19.rst):

> The base template for `std::char_traits` has been removed in LLVM 19. If you are using `std::char_traits` with types other than `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t` or a custom character type for which you specialized `std::char_traits`, your code will stop working. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior while going undetected.

Fixes the following eror:
```
llvm-project/libcxx/include/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,
      |                                          ^
llvm-project/libcxx/include/__type_traits/is_constructible.h:24:79: note: in instantiation of template class 'std::basic_string_view<unsigned char>' requested here
   24 | struct _LIBCPP_TEMPLATE_VIS is_constructible : public integral_constant<bool, __is_constructible(_Tp, _Args...)> {};
      |                                                                               ^
folly/Traits.h:694:24: note: in instantiation of template class 'std::is_constructible<std::basic_string_view<unsigned char>, const unsigned char *const &, unsigned long>' requested here
  694 |     : std::conditional<T::value, Conjunction<TList...>, T>::type {};
      |                        ^
folly/Range.h:647:9: note: in instantiation of template class 'folly::Conjunction<std::is_constructible<std::basic_string_view<unsigned char>, const unsigned char *const &, unsigned long>, std::is_constructible<folly::Range<const char *>, std::basic_string_view<unsigned char>>>' requested here
  647 |       : Conjunction<
      |         ^
folly/Range.h:666:16: note: in instantiation of template class 'folly::Range<const unsigned char *>::IsConstructibleViaStringView<folly::Range<const char *>>' requested here
  666 |               !IsConstructibleViaStringView<Tgt>::value,
      |                ^
folly/Range.h:668:22: note: while substituting prior template arguments into non-type template parameter [with Tgt = folly::Range<const char *>]
  668 |   constexpr explicit operator Tgt() const noexcept(
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  669 |       std::is_nothrow_constructible<Tgt, Iter const&, size_type>::value) {
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  670 |     return Tgt(b_, walk_size());
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  671 |   }
      |   ~
folly/Range.h:1665:19: note: while substituting deduced template arguments into function template 'operator type-parameter-0-0' [with Tgt = folly::Range<const char *>, $1 = (no value)]
 1665 |       StringPiece(haystack), StringPiece(needles));
      |                   ^
llvm-project/libcxx/include/__string/char_traits.h:45:8: note: template is declared here
   45 | struct char_traits;
      |        ^
```

Reviewed By: thevinster

Differential Revision: D62275652

fbshipit-source-id: fc4288c7113f7a76b96f21d4d3766d75bec11988

folly/Range.h

@@ -638,7 +638,7 @@ class Range {
   template <typename ValueType>
   struct StringViewType //
       : std::conditional<
-            std::is_trivial<ValueType>::value,
+            detail::range_is_char_type_v_<Iter>,
             std::basic_string_view<ValueType>,
             NotStringView> {};