fix(cxx_indexer): always add . to the VFS (#3715)
fix(cxx_indexer): always add . to the VFS (#3715)

clang's Preprocessor::LookupFile always expects FileMgr.getDirectory(".")
to succeed, so we should make sure there's a directory there.
kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe