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

[Timm Baeder via cfe-commits]

tbaederr wrote:

This all looks like welcome improvements to me, but I'll let someone else make the final call. There are currently C++ meetings though, so some people might not be as available.

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