Teach FileCheck to handle trailing CHECK-NOT patterns.

Teach FileCheck to handle trailing CHECK-NOT patterns.

A CHECK-NOT pattern without a following CHECK pattern simply checks that the
pattern doesn't match before the end of the input file.

You can even have only CHECK-NOT patterns to check that strings appear nowhere
in the input file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116592 91177308-0d34-0410-b5e6-96231b3b80d8