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

[Shivam Gupta via llvm-commits]

xgupta wrote:

> I see. This made it seem like it was already enabled by default:
> 
> ```
> .. option:: --diff <value>
> 
>   Controls how mismatches between the check patterns and the input are
>   reported. The default is ``unidiff``, which shows the unified diff output.
> ```
> 
>  ```
> DiffFormatType DiffFormat = DiffFormatType::Unified;
> ```

I see, GlobalISel test cases failure were related then. Kept it None.

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