[clang] [verify] Improve the error messages with multiple active prefixes (PR #126068)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I think this is a distinct improvement, and don't see anything to be concerned about in the code.  

Aaron is away at the WG14 meeting this week so don't expect a review from him anytime soon. That said, please give others ~24 hrs before merging.

https://github.com/llvm/llvm-project/pull/126068