[llvm] [CodeGen] Expose the extensibility of PassConfig to plugins (PR #139059)

[Stefan Gränitz via llvm-commits]

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

Great, LGTM. @arsenm What do you think?

Let's keep the PR open for a few more days so everyone has the chance to add their notes. Otherwise I will merge it towards the end of the week.

https://github.com/llvm/llvm-project/pull/139059