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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I don't think you can reasonably test this with a lit test, you have to add this as a proper build target in llvm/examples similar to Bye.

A lit test could then drive the load of that plugin, but the lit test can't contain the pass itself 

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