Support for OCaml native debugging

This introduces basic support for debugging OCaml binaries.
Use of the native compiler with DWARF emission support (see
https://github.com/ocaml/ocaml/pull/574) is required.

Available variables are considered as 64 bits unsigned integers,
their interpretation will be left to a OCaml-made debugging layer.

Differential revision: https://reviews.llvm.org/D22132

llvm-svn: 277443