Adding Ocaml bindings for the bitreader as requested by Sarah

Thompson. Usage should be something like this:

open Llvm
open Llvm_bitreader

match read_bitcode_file fn with
  | Bitreader_failure msg ->
      prerr_endline msg
  | Bitreader_success m -> 
      ...;
      dispose_module m

Compile with: ocamlc llvm.cma llvm_bitreader.cma
              ocamlopt llvm.cmxa llvm_bitreader.cmxa

llvm-svn: 44824