[llvm] [OCaml] Build OCaml bindings using Dune (PR #67272)

[via llvm-commits]

alan-j-hu wrote:

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 `libLLVMXYZ.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 `libLLVMXYZ.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
```

https://github.com/llvm/llvm-project/pull/67272