[libcxx] Fix typo in spelling of 'sentinel'. NFC.

[ldionne]

Ahah, I had spotted that one too! Thanks for fixing.

libcxx/include/filesystem

 };
 
 template <class _ECharT>
 typename enable_if<__can_convert_char<_ECharT>::value, bool>::type
 __is_separator(_ECharT __e) {
   return __e == _ECharT('/');
 }
 
-struct _NullSentinal {};
+struct _NullSentinel {};
 
 template <class _Tp>
 using _Void = void;
 
 template <class _Tp, class = void>
 struct __is_pathable_string : public false_type {};
 
 template <class _ECharT, class _Traits, class _Alloc>

 template <class _Source, class _ECharT, class _UPtr>
 struct __is_pathable_char_array<_Source, _ECharT*, _UPtr, true>
     : __can_convert_char<typename remove_const<_ECharT>::type> {
   using _Base = __can_convert_char<typename remove_const<_ECharT>::type>;
 
   static _ECharT const* __range_begin(const _ECharT* __b) { return __b; }
   static _ECharT const* __range_end(const _ECharT* __b) {
     using _Iter = const _ECharT*;
-    const _ECharT __sentinal = _ECharT{};
+    const _ECharT __sentinel = _ECharT{};
     _Iter __e = __b;
-    for (; *__e != __sentinal; ++__e)
+    for (; *__e != __sentinel; ++__e)
       ;
     return __e;
   }
 
   static _ECharT __first_or_null(const _ECharT* __b) { return *__b; }
 };
 
 template <class _Iter, bool _IsIt = __is_cpp17_input_iterator<_Iter>::value,
           class = void>
 struct __is_pathable_iter : false_type {};
 
 template <class _Iter>
 struct __is_pathable_iter<
     _Iter, true,
     _Void<typename __can_convert_char<
         typename iterator_traits<_Iter>::value_type>::__char_type> >
     : __can_convert_char<typename iterator_traits<_Iter>::value_type> {
   using _ECharT = typename iterator_traits<_Iter>::value_type;
   using _Base = __can_convert_char<_ECharT>;
 
   static _Iter __range_begin(_Iter __b) { return __b; }
-  static _NullSentinal __range_end(_Iter) { return _NullSentinal{}; }
+  static _NullSentinel __range_end(_Iter) { return _NullSentinel{}; }
 
   static _ECharT __first_or_null(_Iter __b) { return *__b; }
 };
 
 template <class _Tp, bool _IsStringT = __is_pathable_string<_Tp>::value,
           bool _IsCharIterT = __is_pathable_char_array<_Tp>::value,
           bool _IsIterT = !_IsCharIterT && __is_pathable_iter<_Tp>::value>
 struct __is_pathable : false_type {
static void __append_range(string& __dest, _Iter __b, _Iter __e) {
     if (__b == __e)
       return;
     basic_string<_ECharT> __tmp(__b, __e);
     _Narrower()(back_inserter(__dest), __tmp.data(),
                 __tmp.data() + __tmp.length());
   }
 
   template <class _Iter>
-  static void __append_range(string& __dest, _Iter __b, _NullSentinal) {
+  static void __append_range(string& __dest, _Iter __b, _NullSentinel) {
     static_assert(!is_same<_Iter, _ECharT*>::value, "Call const overload");
-    const _ECharT __sentinal = _ECharT{};
-    if (*__b == __sentinal)
+    const _ECharT __sentinel = _ECharT{};
+    if (*__b == __sentinel)
       return;
     basic_string<_ECharT> __tmp;
-    for (; *__b != __sentinal; ++__b)
+    for (; *__b != __sentinel; ++__b)
       __tmp.push_back(*__b);
     _Narrower()(back_inserter(__dest), __tmp.data(),
                 __tmp.data() + __tmp.length());
   }
 
   template <class _Source>
   static void __append_source(string& __dest, _Source const& __s) {
     using _Traits = __is_pathable<_Source>;
struct _PathCVT<char> {
 
   template <class _Iter>
   static typename enable_if<__is_cpp17_forward_iterator<_Iter>::value>::type
   __append_range(string& __dest, _Iter __b, _Iter __e) {
     __dest.__append_forward_unsafe(__b, __e);
   }
 
   template <class _Iter>
-  static void __append_range(string& __dest, _Iter __b, _NullSentinal) {
-    const char __sentinal = char{};
-    for (; *__b != __sentinal; ++__b)
+  static void __append_range(string& __dest, _Iter __b, _NullSentinel) {
+    const char __sentinel = char{};
+    for (; *__b != __sentinel; ++__b)
       __dest.push_back(*__b);
   }
 
   template <class _Source>
   static void __append_source(string& __dest, _Source const& __s) {
     using _Traits = __is_pathable<_Source>;
     __append_range(__dest, _Traits::__range_begin(__s),
                    _Traits::__range_end(__s));