[lld] [lld][COFF][LTO] Implement /opt:emitllvm option (PR #66964)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I think the first commit looks good to go at this point; do you want to make a separate PR for that, that we can approve right away?

https://github.com/llvm/llvm-project/pull/66964