[vim] Recognize more FileCheck comments

Summary:
Currently vim syntax highlighting recognizes 'CHECK:' as a special
comment, but not CHECK-DAG, CHECK-NOT and other CHECKs. This patch
adds rules for these comments.

Reviewers: chandlerc, compnerd, rogfer01

Reviewed By: rogfer01

Subscribers: rogfer01, llvm-commits

Differential Revision: https://reviews.llvm.org/D43289

llvm-svn: 325599