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

[Nico Weber via cfe-commits]

nico wrote:

FWIW, clang has a warning like that too, it's `-Wmismatched-tags` (part of `-Wmost`, which is part of `-Wall`).

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