[lld] [lld][ELF] Demote symbols in discarded sections to Undefined. (PR #68777)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I think we need a different approach to implement symbol demoting. I re-implemented the stuff in https://github.com/maskray/llvm-project/tree/lld-symbols-in-discard and added you as a co-author :)

https://github.com/llvm/llvm-project/pull/68777