[clangd] Fix remote index build failures due to lack of proto dependency

Previous attempt (15f6bad6) introduced
add_dependencies but unfortunately it does not actually add a dependency
between RemoteIndexProto and RemoteIndexServiceProto. This is likely due
to some requirements of it that clang_add_library violates.

As a workaround, we will link RemoteIndexProto library to
RemoteIndexServiceProto which is logical because the library can not be
without linking to RemoteIndexProto anyway.