[clangd] NFC: Use deprecated grpc++ headers for compatibility

[kbobyrev]

Summary

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.

https://packages.ubuntu.com/bionic/amd64/libgrpc++-dev/filelist

clang-tools-extra/clangd/index/remote/Client.cpp

 //===--- Client.cpp ----------------------------------------------*- C++-*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-#include <grpcpp/grpcpp.h>
+#include <grpc++/grpc++.h>
 
 #include "Client.h"
 #include "Index.grpc.pb.h"
 #include "index/Serialization.h"
 #include "marshalling/Marshalling.h"
 #include "support/Logger.h"
 #include "support/Trace.h"
 

clang-tools-extra/clangd/index/remote/server/Server.cpp

 //===--- Server.cpp - gRPC-based Remote Index Server  ---------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "index/Index.h"
 #include "index/Serialization.h"
 #include "index/remote/marshalling/Marshalling.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Signals.h"
 
-#include <grpcpp/grpcpp.h>
-#include <grpcpp/health_check_service_interface.h>
+#include <grpc++/grpc++.h>
+#include <grpc++/health_check_service_interface.h>
 
 #include "Index.grpc.pb.h"
 
 namespace clang {
 namespace clangd {
 namespace remote {
 namespace {