Revert r374931 "[llvm-objdump] Use a counter for llvm-objdump -h instead of the section index."

This broke llvm-objdump in 32-bit builds, see e.g.
http://lab.llvm.org:8011/builders/clang-cmake-armv7-quick/builds/10925

> Summary:
> When listing the index in `llvm-objdump -h`, use a zero-based counter instead of the actual section index (e.g. shdr->sh_index for ELF).
>
> While this is effectively a noop for now (except one unit test for XCOFF), the index values will change in a future patch that filters certain sections out (e.g. symbol tables). See D68669 for more context. Note: the test case in `test/tools/llvm-objdump/X86/section-index.s` already covers the case of incrementing the section index counter when sections are skipped.
>
> Reviewers: grimar, jhenderson, espindola
>
> Reviewed By: grimar
>
> Subscribers: emaste, sbc100, arichardson, aheejin, arphaman, seiya, llvm-commits, MaskRay
>
> Tags: #llvm
>
> Differential Revision: https://reviews.llvm.org/D68848

llvm-svn: 375088