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

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.