add a comment explainin why std::enable_if is used on this method

Author: andrurogerz

llvm/include/llvm/ADT/ilist_node.h

@@ -149,6 +149,11 @@ class ilist_node_impl
   ///
   /// This requires sentinel tracking to be explicitly enabled.  Use the
   /// ilist_sentinel_tracking<true> option to get this API.
+  ///
+  /// Rather than using static_assert to enforce the API is not called when
+  /// configured with is_sentinel_tracking_explicit=false, the method is
+  /// conditionally provided using std::enable_if.  This way, clients of
+  /// ilist_node_impl can be fully instantiated for DLLExport on Windows.
   template <typename T = OptionsT>
   std::enable_if_t<T::is_sentinel_tracking_explicit, bool> isSentinel() const {
     return node_base_type::isSentinel();