[c-index-test] When building with BUILD_CLANG_ONLY=YES, include c-index-test.

It is part of libclang and has other uses besides running the clang tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167478 91177308-0d34-0410-b5e6-96231b3b80d8