Fix cindex.py compatibility with older libclang.so

The issue is that we were calling clang_getCompletionBriefComment
unconditionally.  New we check if this function is available before calling it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164464 91177308-0d34-0410-b5e6-96231b3b80d8