vim: special case the CHECK prefix
vim: special case the CHECK prefix

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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@285927 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm