[LLVM-C] Add Section and Symbol Iterator Accessors for Object File Binaries

Summary: This brings us to full feature parity with the old API, so I've deprecated it and updated the tests.  I'll do a follow-up patch to do some more cleanup and documentation work in this header.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60407

llvm-svn: 358037