[llvm] [FileCheck]: Fix diagnostic for trailing CHECK-NOT (PR #78412)

[Thomas Preud'homme via llvm-commits]

RoboTux wrote:

I feel what we need is to change DagNotStrings to be a vector of FileCheckString instead of a vector of Pattern since it's FileCheckString that associate a Prefix, Loc and Pattern. All we need then is to create FileCheckString for CHECK-NOTs and CHECK-DAGs with an empty DagNotStrings for them.

Does that make sense?

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