[clang] [clang][ExprConst] Fix second arg of __builtin_{clzg,ctzg} not always being evaluated (PR #86742)

[via cfe-commits]

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@@ -12425,12 +12430,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
 
+    std::optional<APSInt> Fallback;
+    if (BuiltinOp == Builtin::BI__builtin_ctzg && E->getNumArgs() > 1) {
+      APSInt FallbackTemp;
+      if (!EvaluateInteger(E->getArg(1), FallbackTemp, Info))
+        return false;
+      Fallback = FallbackTemp;
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overmighty wrote:

I guess using a `bool HasFallback` instead of `std::optional` would be more efficient but defaulting to `std::optional` feels like better practice.

https://github.com/llvm/llvm-project/pull/86742