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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

This seems ok, but I feel like what you really want is `PB.buildThinLTOPreLinkDefaultPipeline(OL)` [here](https://github.com/llvm/llvm-project/blob/f804e2badf30321121df4d0d7df8e32e10f134cc/llvm/lib/LTO/LTOBackend.cpp#L336).

IIUC, you want optimizations, but more like the ThinLTO pre-link phase (e.g. we don't run `EliminateAvailableExternallyPass` for the [ThinLTO pre-link pipeline](https://github.com/llvm/llvm-project/blob/ed4bdb86b084bf633770136d005426adeeb2cd57/llvm/lib/Passes/PassBuilderPipelines.cpp#L1344) for basically the same reasons you want to skip it) that don't drop information. And apparently there's already precedent for something like this with `Conf.UseDefaultPipeline`. Thoughts on that approach instead?

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