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

[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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NagyDonat wrote:

I see your point, but `CanBeNegative` is also somewhat incorrect here because at this point (in the most recent `State`) the value of the symbol cannot be negative. I think I'll use something like `AlsoMentionUnderflow` with a comment that explains its meaning.

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