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

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

> I'm not sure what you mean here. How are you invoking your LTO link? You can pass internal options via the linker or other tools that are invoking LTO. E.g. for an lld LTO link: -Wl,-mllvm,-enable-elim-avail-extern=false (making up a possible name for the internal option).

Our tool uses the LLVM libraries directly, similar to `llvm-lto2` and `lld`, i.e. we create an `LTO` object, add `InputFiles`, run the process, and so on, and this is just one of the transformations the tool does. It is not a script or anything like that.

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