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

[Marlus Cadanus da Costa via cfe-commits]

marlus wrote:

I have read the LLVM contribution policies, including the LLVM AI Tool Use Policy. AI tool usage (GitHub Copilot / Claude Sonnet 4.6) has been noted in the PR description.

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