[clang] [Clang] Implement diagnostics for why is_final is false (PR #154863)

[Samarth Narang via cfe-commits]

snarang181 wrote:

> Probably still worth a release note, else lgtm.

Thanks @erichkeane for reviewing. Linking https://github.com/llvm/llvm-project/pull/144161#discussion_r2166578091 where @cor3ntin mentioned we should have a single entry in `ReleaseNotes` for the whole feature of implementing diagnostics for type traits evaluating to false.  

https://github.com/llvm/llvm-project/pull/154863