[index] Remove redundancy between symbol kind and language

Condense the ObjCKIND and CXXKIND options into just KIND, since the
language was already specified on a per-symbol basis and this
information was redundant. This only changes the internal
representation; naturally the libclang interface remains the same.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264423 91177308-0d34-0410-b5e6-96231b3b80d8