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

[Nico Weber via cfe-commits]

nico wrote:

> Sounds like if we want strict MSVC bug-for-bug conformance, we should change that to use the first declaration then.

We don't want bug-for-bug conformance, but we do want to be ABI compatible. So we do have to undo the regression in behavior here. (I think this was covered by tests even, right?) So we'd appreciate a fix 🙂

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