[lld] [lld-macho] Use parallel algorithms in favor of `ThreadPool` (PR #99471)

[Nico Weber via llvm-commits]

https://github.com/nico approved this pull request.

This resolves https://reviews.llvm.org/D137368#3987994, which is great.

I'm a big fan of this patch, thanks!

https://github.com/llvm/llvm-project/pull/99471