[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, but we should ensure the initial version is stable and free of major issues before merging.

I suggest adding FILECHECK_OPTS to a `.github/workflows` file in a draft, "DO NOT SUBMIT" patch, to test correctness.

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