[clang] [clang] Generate note on declaration for nodiscard-related attributes (PR #112289)

[via cfe-commits]

Sirraide wrote:

I.e. could we get it to emit something like:
```console
<source>:10:5: warning: ignoring return value of type 'S<int>' declared with 'nodiscard' attribute [-Wunused-result]
   10 |     getS(2);
      |     ^~~~ ~
1 warning generated.
```

https://github.com/llvm/llvm-project/pull/112289