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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

insertPass is a bad API (and probably going away in the new PM?). Should probably have a set of insert points like the new pass manager does for middle end passes. Also, needs tests / a sample and documentation 

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