[clangd] Provide help text to users who run `clangd` in a terminal.

[hokein]

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

430

do we want to exit clangd for this case?

[sammccall]

Oops, forgot to hit send.

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

430

I'd prefer not to, it's only "somewhat likely" the user is confused - there's a chance they're going to e.g. paste in LSP messages.

Also behaving *completely* differently when connected to a TTY violates expectations and can make debugging hard. Changing the log output seems fine, refusing to speak the protocol doesn't.

[hokein]

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

430

Sounds good.

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

if (EC) {
       Tracer = trace::createJSONTracer(*TraceStream, PrettyPrint);
     }
   }
 
   llvm::Optional<trace::Session> TracingSession;
   if (Tracer)
     TracingSession.emplace(*Tracer);
 
+  // If a user ran `clangd` in a terminal without redirecting anything,
+  // it's somewhat likely they're confused about how to use clangd.
+  // Show them the help overview, which explains.
+  if (llvm::outs().is_displayed() && llvm::errs().is_displayed())
+    llvm::errs() << Overview << "\n";
   // Use buffered stream to stderr (we still flush each log message). Unbuffered
   // stream can cause significant (non-deterministic) latency for the logger.
   llvm::errs().SetBuffered();
   StreamLogger Logger(llvm::errs(), LogLevel);
   LoggingSession LoggingSession(Logger);
   // Write some initial logs before we start doing any real work.
   log("{0}", clang::getClangToolFullVersion("clangd"));
   {