a missing include from CLog.h.

CLog.h referenced most of the core libclang types but never directly
included Index.h that provides them. Previously it got lucky and other
headers were always included first but with the sorting it ended up
first in one case and stopped compiling. Adding the Index.h include
fixes it right up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202810 91177308-0d34-0410-b5e6-96231b3b80d8