[clang] [Clang] Fix inverted diagnostic location for duplicate default labels (PR #180447)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > The answer is that the order is inconsistent between language modes:
> > In C: The list iterates forward.
> > In C++: The list iterates in reverse order.
> 
> We can dig deeper. Why the order is inconsistent between language modes?

Yep!  Also, what sort of work does AST Dump do to 'get it right'? It always dumps them in the right order: https://godbolt.org/z/as1ddMnva

https://github.com/llvm/llvm-project/pull/180447