[lld] [ELF] Demote symbols in /DISCARD/ discarded sections to Undefined (PR #69295)

[Bevin Hansson via llvm-commits]

https://github.com/bevin-hansson commented:

Looks good to me, though I don't know how it might affect other parts of the linking than what we use downstream.

https://github.com/llvm/llvm-project/pull/69295