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.

llvm-svn: 285927
diff --git a/llvm/utils/vim/syntax/llvm.vim b/llvm/utils/vim/syntax/llvm.vim
index 8fb2236..3a2b828 100644
--- a/llvm/utils/vim/syntax/llvm.vim
+++ b/llvm/utils/vim/syntax/llvm.vim
@@ -192,6 +192,7 @@
 syn match  llvmSpecialComment /;\s*PR\d*\s*$/
 syn match  llvmSpecialComment /;\s*REQUIRES:.*$/
 syn match  llvmSpecialComment /;\s*RUN:.*$/
+syn match  llvmSpecialComment /;\s*CHECK:.*$/
 syn match  llvmSpecialComment /;\s*XFAIL:.*$/
 
 if version >= 508 || !exists("did_c_syn_inits")