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

[Erich Keane via cfe-commits]

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@@ -4365,6 +4341,7 @@ def CIR_FuncOp : CIR_Op<"func", [
     TypeAttrOf<CIR_FuncType>:$function_type,
     UnitAttr:$builtin,
     UnitAttr:$coroutine,
+    UnitAttr:$flattened_coroutine,
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erichkeane wrote:

Wonder if we could/should have a 'coroutine-ness' type attr instead, a tri-state of: "None, Whatever, Flattened" kinda thing.  WDYT? 

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