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

[Shivam Gupta via llvm-commits]

xgupta wrote:

> 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.

Sure, Let me rebase the remaining patches, get them merge so phase 1 can be share with community to evaluate.   

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