[clang] [clang][sema] Fix crash on class template instantiation when a member variable partial specialization has an invalid primary template (PR #202006)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I am not talking about RecoveryExpr here.

Keeping the AST valid is much better, as that's easier to reason about, specially when keeping it valid is very simple, such as this case.

I knew I saw this before, because I recently reviewed https://github.com/llvm/llvm-project/pull/200161 and that's fixing the same bug as here. But that patch is much better than this one, because it doesn't rely on checking the invalid bit.

https://github.com/llvm/llvm-project/pull/202006