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

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

https://lab.llvm.org/buildbot/#/builders/10/builds/9136 passed. So far, other fails are bots that flake or fail reliably.

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