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

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Tcc100 wrote:

I added a test case and some docs, which tuned out to be more challenging than expected, since I couldn't find any test case for the plugin system. The lit test in this PR requires the current llvm headers, which are not provided by the lit system, thus, it currently only works if you provide the absolute path in your system. I'd appreciate any pointers on how to implement this correctly.

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