[clang] [llvm] [clang][nullability] Don't return null fields from `getReferencedDecls()`. (PR #94983)

[Yitzhak Mandelbaum via cfe-commits]

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

Thanks! I'm once again struck by how much this project would benefit from Nullability annotations/checking.

https://github.com/llvm/llvm-project/pull/94983