[clang] [Clang] fix diagnostic to correctly handle singular and plural cases for redundant qualifiers on base class type (PR #125943)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane thanks. I've changed the title/commit message.

You did not unfortunately, so the commit has just:

```[Clang] fix diagnostic to correctly handle singular and plural cases for redundant qualifiers on base class type 

Fixes https://github.com/llvm/llvm-project/issues/125942
```

PLEASE next time make sure the description of the PR (and thus the final commit message) describes the problem, and what you're doing to solve it.

https://github.com/llvm/llvm-project/pull/125943