[OCaml] Introduce an llmdkind abstract type.

Patch by Gabriel Radanne.

While this commit technically breaks API, no code should have supplied
the integer IDs directly, and thus no code should break.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210395 91177308-0d34-0410-b5e6-96231b3b80d8