[clang] [clang][Sema] fix Crash when lookup finds UnresolvedUsingValueDecl followed by '<' (PR #190322)

[Erich Keane via cfe-commits]

https://github.com/erichkeane requested changes to this pull request.

This does a horrifying amount of extra 'reflow'-ing' for what really amounts to the `Result.Empty` check also needing to ensure it is a template decl.

https://github.com/llvm/llvm-project/pull/190322