Disable the c-index-test build target

It depends on libXML and thus cannot be built in our Linux sysroot.
Using -DLLVM_ENABLE_LIBXML=OFF doesn't work (although it was
fixed upstream in the monorepo).
Rather than fixing this "properly" or even just  hacking out the LibXML check,
hack out c-index-test from the build entirely, thus making it faster.