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

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I think as written, this is correct.  We are in an error-condition, there isn't really a way for us to recover from this/instantiate the invalid variable template.

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