[llvm] [lldb] Add synthetic formatter for llvm::Expected (PR #118758)

[via llvm-commits]

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

LGTM.  This will look natural in printing, and programmatically checking the child name to see if you have a value or an error is straightforward.

https://github.com/llvm/llvm-project/pull/118758