message: Add copy constructor and assignment to XlaOp.
diff: @@ -56,6 +56,9 @@ class XlaOp {
  }
  ~XlaOp() = default;
  + XlaOp(const XlaOp& other) = default;
  + XlaOp& operator=(const XlaOp& other) = default;
  +
  // Precondition: !IsUninitialized().
  //
  // It's very common to do foo.builder()->bar(). Without this precondition, if
  