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

[Kim Gräsman via cfe-commits]

kimgr wrote:

Did this PR intentionally break the `RecursiveASTVisitor` API? The new `TraverseQualifier` argument through the whole `Traverse.*Type` hierarchy makes all CRTP overrides not be called, unless I'm missing something.

It's a bit hard to test, because the NNS changes also break a lot of code in IWYU, so it's tricky to play with things in isolation.

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