[clang] [clang][CodeGen] Ensure consistent `mustprogress` attribute emission across all paths (PR #71452)

[Antonio Frighetto via cfe-commits]

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@@ -1482,6 +1477,11 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
   } else
     llvm_unreachable("no definition for emitted function");
 
+  // This is checked after emitting the function body so we know if there
+  // are any permitted infinite loops.
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antoniofrighetto wrote:

Comment was added in https://github.com/llvm/llvm-project/commit/ac73b73c16526c9e51943759ea6cab285a57e33f. Perhaps something along these lines could work better?
```cpp
// This is checked after emitting the function body, so as to ensure
// that the function adheres to the forward progress guarantee,
// which is required by certain optimizations.
```

https://github.com/llvm/llvm-project/pull/71452