[clang] [lld] [lld][LTO] Respect `-fno-slp-vectorize` for the LTO pipeline (PR #201585)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay requested changes to this pull request.

Adding control options to PipelineTuningOptions members is cumbersome and every lld port might need it. Linker options have stronger stability guarantee while we shouldn't guarantee much here.

Can we use a `cl::opt` option? Can we make `ld.lld -mllvm=-vectorize-slp=0` disable the vectorizer, if it doesn't today?

https://github.com/llvm/llvm-project/pull/201585