[clang] [Clang][Sema] Fix crash when MS dependent base class lookup occurs in an incomplete context (PR #83024)

[Erich Keane via cfe-commits]

erichkeane wrote:

Side note: In the future can you be more descriptive with follow up commit messages, isntead of just `[FOLD]`.  That doesn't really tell me whether I need to re-review/give context, particularly since I do much of that triage in email.

As far as this patch, feel free to commit when CI gets to a point where you're not breaking it.

https://github.com/llvm/llvm-project/pull/83024