Revert xhpast changes that impacted builds under Bison 2.3

Summary:
Fixes T9753. Changes some time ago (in D13970 + D13974) improved XHPAST build compile-time warning behavior under Bison 3.

However, macOS still ships with Bison 2.3 and these changes prevent XHPAST from building with Bison 2.3. The changes didn't introduce version detection, so Bison 2.3 builds fail somewhat mysteriously without obvious next steps.

It's relatively easy to install Bison 3 on macOS via Homebrew, but the Bison 3 changes aren't terribly substantive and XHPAST doesn't actually depend on any Bison 3 features, so just return to Bison 2.3 for now.

It would be reasonable to undo this again and retarget Bison 3 in the future, but ideally we should wait until macOS ships with Bison 3 or we have a specific reason to bump the minimum required version to 3. If/when we do, we should version-detect Bison and raise a clear error message.

Test Plan: Built xhpast under Bison 2.3 on a default macOS install using "make cleanall && make install".

Maniphest Tasks: T9753

Differential Revision: https://secure.phabricator.com/D21063