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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

It would be hard for us to document this as an intended interface, but leave that assert in the way, even if you found yourself a way to avoid it. This wouldn't look very polished.

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