[llvm] [LTO] Enable adding custom pass instrumentation callbacks (PR #71268)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

> Ah got it. The typical way then would be to have a flag in the PipelineTuningOptions struct that can be set from your tool and queried by the PM builder.

A setting in the `PipelineTuningOptions` would be a good variant, especially because the unwanted pass would not even be instantiated and added to the pipeline. However, a setting like `EnableEliminateAvailableExternallyPass` seems like too ad hoc a solution, and I'd prefer to have a bit more general approach, as there may be other passes in the future that we (or others) may want to disable for a specific task.

https://github.com/llvm/llvm-project/pull/71268