[clang] [analyzer] Mention possibility of underflow in array overflow errors (PR #84201)

[Balázs Kéri via cfe-commits]

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@@ -603,6 +611,8 @@ void ArrayBoundCheckerV2::performCheck(const Expr *E, CheckerContext &C) const {
     auto [WithinUpperBound, ExceedsUpperBound] =
         compareValueToThreshold(State, ByteOffset, *KnownSize, SVB);
 
+    bool AssumedNonNegative = SUR.assumedNonNegative();
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balazske wrote:

My first observation was at the text output generation when at the `if` statement `AssumedNonNegative` is true and a message is printed "can be negative or ...", this looks like a bug. Because the same name is used at other places this may clarify the meaning, but a comment would be useful anyway (even if the current name remains).
I would not use a variable, instead call `SUR.assumedNonNegative()` directly, then it is better visible that the value comes from `StateUpdateReporter`.

https://github.com/llvm/llvm-project/pull/84201