[llvm] OCaml bindings: Remove `ctypes` dependency (PR #179417)

[Alan Hu via llvm-commits]

alan-j-hu wrote:

Hmm, would you know the specific problem regarding MSVC and ctypes? I don't want to remove the ctypes dependency, only to find out that the LLVM bindings *also* don't compile with MSVC anyway.

If the dependency on `ctypes` is causing issues, I would be okay with removing it. Then, it would be a reasonable tradeoff to expose the original functions as a separate library depending on the LLVM package, and implemented in pure OCaml code.

However, I still want to keep the unit tests using ctypes and properly test this API, to make sure that future changes to LLVM don't break their functionality. Could you please fix the unit tests, by moving the ctypes-specific code there and preserving the test cases about casting LLVM globals and functions to OCaml values?

https://github.com/llvm/llvm-project/pull/179417