Make atom collections private.

These fields were made protected in r193585. The aim of that change is to
expose these fields to SimpleFileWrapper. Because SimpleFileWrapper class
was removed in r227549, we can make them private.

llvm-svn: 227672

lld/include/lld/Core/Simple.h

@@ -82,7 +82,7 @@ public:
     return make_range(_definedAtoms._atoms);
   }
 
-protected:
+private:
   atom_collection_vector<DefinedAtom>        _definedAtoms;
   atom_collection_vector<UndefinedAtom>      _undefinedAtoms;
   atom_collection_vector<SharedLibraryAtom>  _sharedLibraryAtoms;