[OCaml] Add OCaml API for LLVMGetIndices

Summary:
This patch adds a thin wrapper around LLVMGetNumIndices and
LLVMGetIndices to return the indices of ExtractValue or InsertValue
instructions as an OCaml array. It has not seemed to be necessary to
expose LLVMGetNumIndices separately.

Reviewers: whitequark

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52207

llvm-svn: 342474