[lld] [lld] Add `--lto-emit-asm` and alias `--plugin-opt=emit-llvm` to it (PR #97469)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Thanks for catching this missing alias! I occasionally debug issues with `--lto-emit-asm`. I tried `--lto-emit-llvm`, but I didn't bother adding an alias then.
> 
> Can you add a test to lto/emit-llvm.ll ?
> 
> > Make --lto-emit-llvm to be more consistent with --lto-emit-asm
> 
> I think "add --lto-emit-llvm and make --plugin-opt=emit-llvm" might be a better title. The body can mention that this improves consistency with --lto-emit-asm.

Test added, thanks for the quick review. I also wonder if we shouldn't make LLVM-IR a valid output type from the LTO interface so we don't need to use the hook for it. Maybe make it only valid under monolithic LTO.

https://github.com/llvm/llvm-project/pull/97469