[OCaml] Migrate from naked pointers to prepare for OCaml 5

The OCaml bindings currently return pointers to LLVM objects as-is to
OCaml. These "naked pointers" end up appearing as values of local
variables in OCaml code, stored as part of other OCaml values,
etc. The safety of this design relies on the OCaml runtime system's
ability to distinguish these pointers from pointers to memory on the
OCaml garbage collected heap. In particular, when the OCaml GC
encounters a pointer to memory known to not be part of the OCaml heap,
it does not follow it.

In OCaml 4.02 an optimized "no naked pointers" mode was introduced
where the runtime system does not perform such checks and requires
that no such naked pointers be passed to OCaml code, instead one of
several encodings needs to be used. In OCaml 5, the no naked pointers
mode is now the only mode. This diff uses one of the potential
encodings to eliminate naked pointers, making the LLVM OCaml bindings
compatible with the "no naked pointers" mode of OCaml >= 4.02 ...