[clang] Fix crash when doing special member lookup on forward-declared classes (Fixes llvm/llvm-project#144642) (PR #144828)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Thank you for the patch! Please add tests in `clang/test/` to make sure this fix actually works. Please add a release note in `clang/docs/ReleaseNotes.rst` so that users can know the improvement.

https://github.com/llvm/llvm-project/pull/144828