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

[Andrey Ali Khan Bolshakov via cfe-commits]

bolshakov-a wrote:

They should be avoided not always but in certain circumstances, so there should be a new flag field in the visitor class. Not many new code, TBH, but using `TraverseQualifier` was so tempting...

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