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

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> 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.

We should probably add a FIXME for this.

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