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

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

> I lean on to what Martin says, I am not super excited about re-using `/opt:` for this purpose here (I do find the PR useful otherwise). Maybe `/lldemitasm`, `/lldemitllvm` unless someone has better naming?

I sure don't mind changing the flag to whatever is the best fit. I can wait for a second opinion from @rnk , and we can debate about the spelling all we want :)

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