[clang] [CIR] Implement FlattenCFG for coroutine AwaitOp, CoroBodyOp, and CoReturnOp (PR #203802)

[Erich Keane via cfe-commits]

================
@@ -127,6 +126,8 @@ struct MissingFeatures {
   // Coroutines
   static bool coroAwaitFullExprCleanups() { return false; }
   static bool coroOutsideFrameMD() { return false; }
+  static bool coroutineGroManager() { return false; };
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erichkeane wrote:

```suggestion
  static bool coroutineGetRetObjManager() { return false; };
```

https://github.com/llvm/llvm-project/pull/203802