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

[Henrich Lauko via cfe-commits]

================
@@ -138,14 +138,26 @@ def ZeroAttr : CIR_TypedAttr<"Zero", "zero"> {
 // UndefAttr
 //===----------------------------------------------------------------------===//
 
-def UndefAttr : CIR_TypedAttr<"Undef", "undef"> {
+def CIR_UndefAttr : CIR_TypedAttr<"Undef", "undef"> {
----------------
xlauko wrote:

Same this is being upstreamed in parallel PR https://github.com/llvm/llvm-project/pull/150694 and is related to poison. Please roll back:

```suggestion
def UndefAttr : CIR_TypedAttr<"Undef", "undef"> {
```

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