[lld] [llvm] ELF: Use index 0 for unversioned undefined symbols (PR #168189)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

I'll approve the change, as I think it will be the eventual outcome. Will leave it to your discretion to when the discussion has stabilised. 

https://github.com/llvm/llvm-project/pull/168189