[OCaml] Add OCaml APIs for Invoke arguments and destinations

Summary:
This patch adds OCaml APIs for LLVMGetNormalDest and LLVMGetUnwindDest
on InvokeInsts, as well as LLVMGetNumArgOperands on CallInsts and
InvokeInsts.

Reviewers: whitequark

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52204

llvm-svn: 342433