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

[Shivam Gupta via llvm-commits]

xgupta wrote:

> This implementation is buggy.
> 
> ```
> FILECHECK_OPTS=--diff ninja -C /tmp/Rel check-lld-elf
> ```
> 
> has 1341 failures.

OK that is good way to test the option. I have noted the errors, there are two types of them. Give me few hours, it is fixable. I think no need to revert now since it is off by default.

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