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

[Nick Desaulniers via cfe-commits]

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@@ -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) {
+      if (BuiltinOp == Builtin::BI__builtin_clzg && E->getNumArgs() > 1) {
+        APSInt Fallback;
+        if (!EvaluateInteger(E->getArg(1), Fallback, Info))
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nickdesaulniers wrote:

Could just reuse `Val` here; we don't need extra storage for `Fallback`.

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