[clang] [clang-tools-extra] [clang] Improve nested name specifier AST representation (PR #147835)

[Andrey Ali Khan Bolshakov via cfe-commits]

bolshakov-a wrote:

The assert doesn't hurt us, as I've said. I just asked if it is a "documented way" of working with the new visitor interface. If it not to be avoided, then we will probably just proceed with the `TraverseQualifier` manipulation.

https://github.com/llvm/llvm-project/pull/147835