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

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

I think `/lldemitasm` is good, in that it's less risky of future collisions with `link.exe`, but at the same time I find it hard on the eyes.
I wish we grouped all those `lld-link` exclusive options with a separator similar to `/opt`, something like `/lld:emitllvm` is nicer to read.

Another option is to have an `/outtype` flag that would default to `obj`, but also accept `llvm` and `asm`.
But that is still an `lld-link` exclusive option, and `/lldouttype` is also hard on the eyes.

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