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

[Vinayak Dev via llvm-commits]

vinayakdsci wrote:

@RoboTux I am unable to get the code to detect the NOT prefix where an implicit-check-not occurs. Other than that the solution I have arrived seems to be working fine.

Would it be advisable if I simply set the prefix to IMPLICIT CHECK-NOT: ...?
I think that is a neater diagnostic, but it would require changing some of the existing tests to make the test-suite pass. Other than that, the approach works fine without breakage in the tests.

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