A large number of tests in the LLVM tree use the default (CHECK) prefix
to indicate checked expressions via FileCheck. Highlight it as a
special comment. Although this wont get all the instances of the
checked patters, it is strictly better than the current state.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285927 91177308-0d34-0410-b5e6-96231b3b80d8