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

[Andrey Ali Khan Bolshakov via cfe-commits]

bolshakov-a wrote:

@kimgr, I've just succeeded in building IWYU against this and getting the tests green. However, I want to spend some time on self-review, probably writing additional tests, and so on.

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