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

[Matt Arsenault via llvm-commits]

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@@ -72,6 +74,10 @@ namespace yaml {
 struct MachineFunctionInfo;
 } // namespace yaml
 
+class TargetMachine;
+using PassConfigCallback =
+    std::function<void(TargetMachine &, PassManagerBase &, TargetPassConfig *)>;
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arsenm wrote:

function_ref? 

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