[lld] Support --unresolved-symbols=@<file> option in LLD for ELF (PR #142917)

[Dmitry Nechitaev via llvm-commits]

Nechda wrote:

> I think it would be possible to find these with a separate link step that has a text file containing something like <...>  That second link step could be passed the file <...>

This approach indeed solves the original issue, but we have binaries that take approximately 900 seconds to link, so doubling this time would not be ideal for the user experience.

https://github.com/llvm/llvm-project/pull/142917