[clang] Fix AST IgnoreUnlessSpelledInSource traversal nullptr dereference (PR #146103)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Thank you for the patch! Please add a release note to "clang/docs/ReleaseNotes.rst" so users can know the improvement. Please add a tag to the beginning of the PR title in square brackets like other PRs. E.g. `[clang]`.

https://github.com/llvm/llvm-project/pull/146103