[clang] [clang] Improve diagnostic on [[nodiscard]] attribute (PR #112521)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This seems reasonable, I have 3 comments, which are a request to release note that we're changing the preference here, plus some improvements to the diagnostic.

https://github.com/llvm/llvm-project/pull/112521