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

[Erich Keane via cfe-commits]

erichkeane wrote:

> I think this is fine the way it is, but I’ll leave it to @erichkeane to approve this seeing as I proposed this approach here iirc.
> 
> Side note: please try to avoid force-pushing in the future as that makes it really annoying to try and figure out what has changed since the last review. We always squash on merge anyway, so a clean commit history on a pr is irrelevant.

I'm away at LLVM dev right now. So won't get a chance to review until next week, but I promise this is one I'll look at asap

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