[clang] Reapply "[Clang] Improve diagnostics for expansion length mismatch" (PR #121044)

[Matheus Izvekov via cfe-commits]

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

LGTM, though I would have preferred to keep the patches separate, it's less of a revert risk.

https://github.com/llvm/llvm-project/pull/121044