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

[Alexander Kornienko via cfe-commits]

alexfh wrote:

And a bit cleaned up (https://godbolt.org/z/q9fGPj54d):
```
class A;
using T = void(__attribute__((swift_async_context)) A *);
T R;
class A;
void fa(__attribute__((swift_async_context)) A *);

bool f() {
  return R == fa;
}
```

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