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

[Baranov Victor via cfe-commits]

vbvictor wrote:

Also, writing "Fixes https://github.com/llvm/llvm-project/issues/144642." at the end of the PR description automatically links the issue to the PR so everyone easily know what problem you solved.

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