[Clang][Sema] Don't say "is declared here" for invalid template locations

If a template is defined via an external AST source, it won't have a
location. When we emit warnings about misusing such templates we
shouldn't emit a "template is declared here" warning with no location,
as that's just confusing.

Reviewers: llvm-beanz, erichkeane, AaronBallman

Reviewed By: erichkeane, AaronBallman

Pull Request: https://github.com/llvm/llvm-project/pull/71264