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

[Jonathan Marriott via cfe-commits]

JonathanMarriott 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]`.

Thanks, I've added a release note in what I think is the appropriate section!

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