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

[Erich Keane via cfe-commits]

erichkeane wrote:

THIS is the important part to figure out here: "switch case list is often built in reverse order during parsing,"

_IS_ it "often", or "always"?  IF "always", handling ordering of the diagnostics is about the right place, just reversing the diagnostic messages.  If "Often", we should figure out WHY that is the case.

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