[clangd] Improve output of --help and --version. NFC.

[ilya-biryukov]

LGTM

clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp

 #include "Trace.h"
 #include "index/SymbolYAML.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
+#include "clang/Basic/Version.h"
 #include <cstdlib>
 #include <iostream>
 #include <memory>
 #include <string>
 #include <thread>
 
 using namespace clang;
 using namespace clang::clangd;
llvm::cl::desc(
         "YAML-format global symbol file to build the static index. Clangd will "
         "use the static index for global code completion.\n"
         "WARNING: This option is experimental only, and will be removed "
         "eventually. Don't rely on it."),
     llvm::cl::init(""), llvm::cl::Hidden);
 
 int main(int argc, char *argv[]) {
   llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
-  llvm::cl::ParseCommandLineOptions(argc, argv, "clangd");
+  llvm::cl::SetVersionPrinter([](llvm::raw_ostream &OS) {
+    OS << clang::getClangToolFullVersion("clangd") << "\n";
+  });
+  llvm::cl::ParseCommandLineOptions(
+      argc, argv,
+      "clangd is a language server that provides IDE-like features to editors. "
+      "\n\nIt should be used via an editor plugin rather than invoked directly."
+      "For more information, see:"
+      "\n\thttps://clang.llvm.org/extra/clangd.html"
+      "\n\thttps://microsoft.github.io/language-server-protocol/");
   if (Test) {
     RunSynchronously = true;
     InputStyle = JSONStreamStyle::Delimited;
     PrettyPrint = true;
   }
 
   if (!RunSynchronously && WorkerThreadsCount == 0) {
     llvm::errs() << "A number of worker threads cannot be 0. Did you mean to "