[clangd] NFC: Use deprecated grpc++ headers for compatibility
Ubuntu 18.04 and older versions do not provide latest gRCP packages and the
ones that are in the repository use deprecated headers. Use these headers to
make builds possible.
Reviewed By: sammccall
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D79487