[lld] [lld][ELF] Add --why-live flag (inspired by Mach-O) (PR #127112)

[Peter Smith via llvm-commits]

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

Thanks for the update. I don't have any more comments. I've marked as approved, but please wait for MaskRay.

https://github.com/llvm/llvm-project/pull/127112