Revert part of "Disallow Archive::child_iterator that don't point to an archive."

This reverts parts of commit r241747. MSVC doesn't like it.

llvm-svn: 241753

llvm/include/llvm/Object/Archive.h

@@ -109,6 +109,7 @@ public:
     Child child;
 
   public:
+    child_iterator() : child(Child(nullptr, nullptr)) {}
     child_iterator(const Child &c) : child(c) {}
     const Child *operator->() const { return &child; }
     const Child &operator*() const { return child; }

llvm/include/llvm/Object/ArchiveWriter.h

@@ -24,10 +24,9 @@ class NewArchiveIterator {
   bool IsNewMember;
   StringRef Name;
 
-  union {
   object::Archive::child_iterator OldI;
+
   StringRef NewFilename;
-  };
 
 public:
   NewArchiveIterator(object::Archive::child_iterator I, StringRef Name);