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

[Vinayak Dev via llvm-commits]

vinayakdsci wrote:

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

Done!

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