[clangd] Fix missing "message" key when responding with unsupported method

[rwols]

Summary

The language server protocol dictates that a ResponseError should have a
[message string][1] describing the error. This adds a simple message to the
error and a simple test.

[1]: https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#response-message

[bkramer]

This looks good. Do you have commit access?

[rwols]

No, I don't have commit access. Feel free to merge.

clang-tools-extra/trunk/clangd/JSONRPCDispatcher.cpp

void JSONOutput::log(const Twine &Message) {
   Logs.flush();
 }
 
 void Handler::handleMethod(llvm::yaml::MappingNode *Params, StringRef ID) {
   Output.log("Method ignored.\n");
   // Return that this method is unsupported.
   writeMessage(
       R"({"jsonrpc":"2.0","id":)" + ID +
-      R"(,"error":{"code":-32601}})");
+      R"(,"error":{"code":-32601,"message":"method not found"}})");
 }
 
 void Handler::handleNotification(llvm::yaml::MappingNode *Params) {
   Output.log("Notification ignored.\n");
 }
 
 void JSONRPCDispatcher::registerHandler(StringRef Method,
                                         std::unique_ptr<Handler> H) {

clang-tools-extra/trunk/test/clangd/unsupported-method.test

+# RUN: clangd -run-synchronously < %s | FileCheck %s
+# It is absolutely vital that this file has CRLF line endings.
+#
+Content-Length: 125
+
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+
+Content-Length: 143
+
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file:///main.cpp","languageId":"cpp","version":1,"text":""}}}
+
+Content-Length: 92
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/jumpInTheAirLikeYouJustDontCare","params":{}}
+# CHECK: {"jsonrpc":"2.0","id":1,"error":{"code":-32601,"message":"method not found"}}
+
+Content-Length: 44
+
+{"jsonrpc":"2.0","id":2,"method":"shutdown"}