[clang] [Clang] Fix offsetof sign-extending unsigned array indices >= 128 (PR #204139)

[Timm Baeder via cfe-commits]

================
@@ -2879,6 +2877,20 @@ bool CastAPS(InterpState &S, CodePtr OpPC, uint32_t BitWidth) {
   return true;
 }
 
+// Cast an AP integer to Sint64, failing constant evaluation if the value is
+// negative or too large to fit (i.e. truncation would change the value).
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool CastNoOverflow(InterpState &S, CodePtr OpPC) {
+  T Source = S.Stk.pop<T>();
+  APSInt Val = Source.toAPSInt();
+  if (Val.isNegative() || Val.getActiveBits() > 63) {
+    return Invalid(S, OpPC);
+  }
----------------
tbaederr wrote:

```suggestion
  if (Val.isNegative() || Val.getActiveBits() > 63)
    return Invalid(S, OpPC);
```

https://github.com/llvm/llvm-project/pull/204139