[clang] [CIR] Add poison attribute (PR #150760)

[Henrich Lauko via cfe-commits]

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@@ -1783,6 +1788,11 @@ static bool isBoolNot(cir::UnaryOp op) {
 //
 // and the argument of the first one (%0) will be used instead.
 OpFoldResult cir::UnaryOp::fold(FoldAdaptor adaptor) {
+  if (auto poison = mlir::dyn_cast_if_present<PoisonAttr>(adaptor.getInput())) {
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xlauko wrote:

```suggestion
  if (auto poison = mlir::dyn_cast_if_present<cir::PoisonAttr>(adaptor.getInput())) {
```

https://github.com/llvm/llvm-project/pull/150760