[FileCheck] - Fix the false positive when -implicit-check-not is used with an...

[FileCheck] - Fix the false positive when -implicit-check-not is used with an unknown -check-prefix.

Imagine we have the following invocation:

`FileCheck -check-prefix=UNKNOWN-PREFIX -implicit-check-not=something`

When the check prefix does not exist it does not fail.
This patch fixes the issue.

Differential revision: https://reviews.llvm.org/D78024