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

[via llvm-commits]

Tcc100 wrote:

@arsenm Thanks for the hint, I added a unit test to check that the callback interface gets called properly and that the example plugin is properly loaded. I also added documentation for the plugin interface to the "Writing an LLVM Pass" docs. What do you think, are we ready to merge?

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