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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Also, why have you yet to look at the suggestion I made about how ast-print manages to get this right without ANY change to parsing?  It seems to me that understanding THAT strategy would inform a less high-touch implementation.

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