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

[Teresa Johnson via llvm-commits]

teresajohnson 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.

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