Add missing dependencies to unittest target
Original patch broke BUILD_SHARED bots and required revert #147947

Reverts llvm/llvm-project#139059

This broke
https://lab.llvm.org/buildbot/#/builders/10/builds/9125/steps/8/logs/stdio

The bot does a SHARED_LIBS=ON build. I can reproduce locally with the
CMake cache file in offload/cmake/caches/AMDGPUBot.cmake as the build
config.

This PR exposes the backend pass config to plugins via a callback.
Plugin authors can register a callback that is being triggered before
the target backend adds their passes to the pipeline. In the callback
they then get access to the `TargetMachine`, the `PassManager`, and the
`TargetPassConfig`. This allows plugins to call
`TargetPassConfig::insertPass`, which is honored in the subsequent
`addPass` of the main backend. We implemented this using the legacy pass
manager since backends still use it as the default.