use is_optional_field_ref instead of is_instantiation_of_v
use is_optional_field_ref instead of is_instantiation_of_v

Summary: `is_instantiation_of_v` doesn't work for alias template.

Reviewed By: yfeldblum

Differential Revision: D33877215

fbshipit-source-id: d8746b19a479ece6161e8278cdc98a866b05947f

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift