[clang] [Clang][Sema] Fix Wswitch-default bad warning in template (PR #76007)

[Aaron Puchert via cfe-commits]

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

For now I guess this is Ok, although I think the better fix would be to diagnose missing or duplicate `default` labels even in the dependent case. Because `default` labels themselves are never dependent.

https://github.com/llvm/llvm-project/pull/76007