[cindex.py]: Speed up lookup of the completion kind

We can directly the number of the kind instead of going through the
completionChunkKindMap.

Formatting time changes from 0.84 to 0.72 seconds.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172899 91177308-0d34-0410-b5e6-96231b3b80d8