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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> I’m very much in favor of this feature,

Actually, I withdraw this -- on reflection, I'm not sure unidiff is the right interface. I'll post a comment to #77257

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