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

[Aaron Ballman via cfe-commits]

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

LGTM, thank you for the fix! Once precommit CI comes back green, I can land the changes on your behalf.

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