Warning: Calling fails_with :llvm is deprecated

I got the following warning message when running `brew update`

```sh
Warning: Calling fails_with :llvm is deprecated!
There is no replacement.
/usr/local/Homebrew/Library/Taps/grpc/homebrew-grpc/Formula/google-protobuf.rb:25:in `<class:GoogleProtobuf>'
Please report this to the grpc/grpc tap!
```
