[clang] fix(95366): enhance cast operation safety with LValue validation (PR #95479)

[via cfe-commits]

https://github.com/Sirraide approved this pull request.

LGTM after addressing one issue.

Looking at the rest of the constant evaluator (and the comments on the issue as well), everything else seems to use `EvaluateInteger`, which makes sure the result of the evaluation is an integer, so this seems like it is the only place where we can wind up with an `AddrLabelDiff`, so rejecting it here makes sense.

https://github.com/llvm/llvm-project/pull/95479