use std::enable_if_t for the return type to compile properly with clang

Author: andrurogerz

llvm/include/llvm/ADT/ilist_node.h

@@ -149,9 +149,8 @@ class ilist_node_impl
   ///
   /// This requires sentinel tracking to be explicitly enabled.  Use the
   /// ilist_sentinel_tracking<true> option to get this API.
-  template <
-      typename = std::enable_if_t<OptionsT::is_sentinel_tracking_explicit>>
-  bool isSentinel() const {
+  template <typename T = OptionsT>
+  std::enable_if_t<T::is_sentinel_tracking_explicit, bool> isSentinel() const {
     return node_base_type::isSentinel();
   }
 };