[llvm] [CodeGen] Expose the extensibility of PassConfig to plugins (PR #139059)

[Antonio Frighetto via llvm-commits]

================
@@ -119,6 +119,9 @@ addPassesToGenerateCode(CodeGenTargetMachineImpl &TM, PassManagerBase &PM,
   PM.add(PassConfig);
   PM.add(&MMIWP);
 
+  for (auto& C : *TargetMachine::TargetPassConfigCallbacks)
----------------
antoniofrighetto wrote:

I think it's fine to use auto when code readability benefits from not spelling it out, which should be the case for callable objects (the container TargetPassConfigCallbacks already hints it is a callback item). That said, since we have an alias for the std::function here, `PassConfigCallback &C` should be fine too. @Tcc100 Could you also please run clang-format here? This should have been `auto &C`.

https://github.com/llvm/llvm-project/pull/139059