[llvm] [FileCheck]: Fix diagnostics for NOT prefixes (PR #78412)

[Thomas Preud'homme via llvm-commits]

RoboTux wrote:

> > LGTM, well done!
> 
> Thanks a lot!
> 
> > @vinayakdsci Do you need me to land this for you?
> 
> It would be really great if you would! This would be my first patch to LLVM. Thanks a lot for your patience and guidance!

I think the commit message need to be rephrased slightly now. How about the following:

> Fixes https://github.com/llvm/llvm-project/issues/70221
> 
> Fix a bug in FileCheck that corrects the error message when multiple prefixes are provided
> through --check-prefixes and one of them is a PREFIX-NOT.
> 
> Earlier, only the first of the provided prefixes was displayed as the erroneous prefix, while the
> actual error might be on the prefix that occurred at the end of the prefix list in the input file.
> 
> Now, the right NOT prefix is shown in the error message.

https://github.com/llvm/llvm-project/pull/78412