[OCaml] Build OCaml bindings using Dune

[alan-j-hu]

This PR still needs some cleanup. Problems of note:

• The function llvm_diagnostic_handler_trampoline in llvm_ocaml.c needs to be marked as static or else the linker will produce an error about non-relocatable code. Even though the error message suggests adding -fPIC, adding this flag doesn't seem to fix the issue. This issue does not happen with the current CMake+OCamlfind build system.
• I currently surround the OCaml doc generation with if(FALSE) endif(). What should I do with this?

The OCaml unit tests currently pass on my computer (if you build ocaml_all and then check-llvm-bindings-ocaml). Therefore, the PR is in a state in which I would appreciate other pairs of eyes to see if this work is on the right track, and if there is possibility that the OCaml bindings could use Dune.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[alan-j-hu]

I realized that when I nuked the build directory and tried again, I can't get the build to work again. The reason is that for each backend, there is some implicit dependency on some other libLLVMSomeTargetMCA.a, such that adding a dependency via CMake add_dependencies does not cause it to be built, but when calling llvm-config --libs <backend>, llvm-config will emit an error that libLLVMSomeTargetMCA.a does not exist.

e.g.

(cd _build/default/backends/AMDGPU && ../../discover.sh AMDGPU)
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMAMDGPUTargetMCA.a
(cd _build/default/backends/RISCV && ../../discover.sh RISCV)
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMRISCVTargetMCA.a
(cd _build/default/backends/X86 && ../../discover.sh X86)
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMX86TargetMCA.a
(cd _build/default/all_backends && ../discover.sh AArch64 AMDGPU ARM AVR BPF Hexagon Lanai LoongArch Mips MSP430 NVPTX PowerPC RISCV Sparc SystemZ VE WebAssembly X86 XCore)
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMAMDGPUTargetMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMRISCVTargetMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMX86TargetMCA.a
(cd _build/default/executionengine && ../discover.sh executionengine mcjit native)
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMMCA.a
llvm-config: error: missing: /home/me/llvm-project/builddune/lib/libLLVMX86TargetMCA.a

Edit: However, building a second time always succeeds. It seems that Dune just ignores the failure to generate a c_library_flags.sexp the second time.

[alan-j-hu]

Fixed the issue.