fix build failure on @mode/opt-hhvm
fix build failure on @mode/opt-hhvm

Summary: Without this diff, we can't use visitation in hhvm. It looks like deduction guide is not supported by `mode/opt-hhvm`.

Reviewed By: vitaut

Differential Revision: D25648315

fbshipit-source-id: d382f02db88694cae14d98066d241e3258baefb7

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