fix build
fix build

Summary:
fbthrift generate errorneous code when cpp type IOBuf pointer is used. Let's use
IOBuf object instead as it is cheap to copy construct and underlying buffer is
anyway shared when objects are copied.

Reviewed By: plapukhov

Differential Revision: D8130040

fbshipit-source-id: 3c109a85951054fed890707f56170b46cfefc917

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly