[llvm] [FileCheck] Add a diff output option for FileCheck (PR #187120)

[Henrik G. Olsson via llvm-commits]

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

I think it would be a good idea to post on discourse letting people know about this new option so that people can try it out and provide feedback/bug reports.

https://github.com/llvm/llvm-project/pull/187120