Use bash script yojson_validator.sh instead of ocaml code

Summary:
Use bash script yojson_validator.sh instead of ocaml code

I found one example where clang_ast_yojson_validator would hang in case of error. This is annoying. Let us piggy-back on the 'converters' and use a trivial bash script.

Test Plan: make -C clang-ocaml test

Reviewers: irp

Reviewed By: irp

Subscribers: mathieubaudet

Differential Revision: https://phabricator.fb.com/D1410628

Tasks: 3964417