[clang] [clang] Implement constexpr support for __builtin_{clzg, ctzg} (PR #86577)

[Nick Desaulniers via cfe-commits]

================
@@ -12367,8 +12368,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const 
CallExpr *E,
                            BuiltinOp != Builtin::BI__lzcnt &&
                            BuiltinOp != Builtin::BI__lzcnt64;
 
-    if (ZeroIsUndefined && !Val)
-      return Error(E);
+    if (!Val) {
----------------
nickdesaulniers wrote:

```c++
if (BuiltinOp == Builtin::BI__builtin_clzg)
 return Success(Val.countl_zero(), E);
return Success(Val.countr_zero(), E);
```

https://github.com/llvm/llvm-project/pull/86577
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