[lld] [lld][COFF]Expose UnifiedLTO and PassPipeline Options (PR #69904)

[via llvm-commits]

Naville wrote:

CrossPosting from Discord:
- I'm not sure how to convert my cpp reproducible to an LTO test case. Kinda need some help on that part.
- Should I also add the remaining LTO options too?

https://github.com/llvm/llvm-project/pull/69904