[llvm] [PassBuilder] Add a pass to LTO postlink step (PR #124053)

[Paul Kirth via llvm-commits]

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

Do we need to update any tests? I seem to recall several pipeline tests that checked the order of passes. I'm not sure if we have any of those for LTO, though, as I don't see them in the test dir.

Assuming CI is happy w/ this, LGTM, but lets confirm w/ @aeubanks or @nikic before landing. Pass ordering has a tendency to have unintended consequences, so I'd feel better if one of them double checked me on that, since we're so close to the branch point.

https://github.com/llvm/llvm-project/pull/124053