[CodeGen] Expose the extensibility of PassConfig to plugins

[Tcc100]

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 as the backend is still using the old pass manager.

The following example shows how plugin authors can use the callback. Since its a callback that is not doing anything without anybody registering it, there shouldn't be any potential harm to the compiler unless a plugin is present.

__attribute__((constructor)) static void initCodeGenPlugin() {
    initializeCodeGenTestPass(*PassRegistry::getPassRegistry());

    TargetMachine::registerTargetPassConfigCallback([](auto &TM, auto &PM, auto *TPC) {
        TPC->insertPass(&GCLoweringID, &CodeGenTest::ID);
    });
}

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write permissions for the repository. In which case you can instead tag reviewers by name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[Tcc100]

@nikic @aeubanks please have a look

[antoniofrighetto]

Missing a test for this?

[arsenm]

insertPass is a bad API (and probably going away in the new PM?). Should probably have a set of insert points like the new pass manager does for middle end passes. Also, needs tests / a sample and documentation

[arsenm]

function_ref?

[arsenm]

no auto (and no &?)