Llvm_bitwriter

`sig external write_bitcode_file : Llvm.llmodule -> string -> bool = "llvm_write_bitcode_file" external write_bitcode_to_fd : ?unbuffered:bool -> Llvm.llmodule -> Unix.file_descr -> bool = "llvm_write_bitcode_to_fd" external write_bitcode_to_memory_buffer : Llvm.llmodule -> Llvm.llmemorybuffer = "llvm_write_bitcode_to_memory_buffer" val output_bitcode : ?unbuffered:bool -> Pervasives.out_channel -> Llvm.llmodule -> bool end`