Work around PR29097 to get the module bots going again.

This replaces an =default constructor with an explicit definition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279522 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 20885de)

Author: adrian-prantl

include/llvm/ADT/ilist.h

@@ -171,15 +171,15 @@ class ilist_iterator
   typedef node_type &node_reference;
 
 private:
-  node_pointer NodePtr = nullptr;
+  node_pointer NodePtr;
 
 public:
   /// Create from an ilist_node.
   explicit ilist_iterator(node_reference N) : NodePtr(&N) {}
 
   explicit ilist_iterator(pointer NP) : NodePtr(NP) {}
   explicit ilist_iterator(reference NR) : NodePtr(&NR) {}
-  ilist_iterator() = default;
+  ilist_iterator() : NodePtr(nullptr) {}
 
   // This is templated so that we can allow constructing a const iterator from
   // a nonconst iterator...