[clang] [APINotes] Diagnose invalid Where.Parameters selectors (PR #209408)

[via cfe-commits]

StoeckOverflow wrote:

Thanks, updated the patch for the remaining cleanup comments.

I left `diagnoseUnused` out-of-line so it stays consistent with `APINotesSelectorDiagnosticReaderState::diagnoseUnused`.

https://github.com/llvm/llvm-project/pull/209408