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

[John Hui via cfe-commits]

https://github.com/j-hui commented:

This looks great, thank you!

I just have a question about the placement of the filecheck statements in `where-parameters-diagnostics.cpp`, but I don't feel strongly about it and am happy with this PR as is!

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