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

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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374931 91177308-0d34-0410-b5e6-96231b3b80d8