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

[Igor Kudrin via llvm-commits]

igorkudrin 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).

Thank you for the idea. The pre-link pipelines seem to suit our needs well. I've prepared another PR for that, #82585.

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