Revert D34192021: Switch to if constexpr
Revert D34192021: Switch to if constexpr

Differential Revision:
D34192021 (https://github.com/facebook/fbthrift/commit/7bfc49cc7ada54041578e5ac9e1bc91c714d9042)

Original commit changeset: febbb6d0494b

Original Phabricator Diff: D34192021 (https://github.com/facebook/fbthrift/commit/7bfc49cc7ada54041578e5ac9e1bc91c714d9042)

fbshipit-source-id: d78b6b69ba66b38523375f1bd86ac123b8d61725

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