[OCaml] Add (get/set)_module_identifer functions

Also:

- Fix a bug that crept in when fixing a buildbot failure in
https://github.com/llvm/llvm-project/commit/f7be9db6220cb39f0eaa12d2af3abedf0d86c303
- Use mlsize_t for cstr_to_string as that is what
caml_alloc_string specifies.

Differential Revision: https://reviews.llvm.org/D98851