[lldb-vscode] Fix handling of RestartRequest arguments.

[jgorbe]

Summary

According to the spec, RestartRequest has an optional "arguments" field, which
is a RestartArguments object. RestartArguments has its own optional "arguments"
field, which is a (LaunchRequestArguments | AttachRequestArguments) object. So
we need to to the "arguments" lookup twice to get to the actual launch
arguments.

[rupprecht]

Is there a test case you can add for this?

[jgorbe]

Added test case.

lldb/tools/lldb-vscode/lldb-vscode.cpp

if (GetString(*g_vsc.last_launch_or_attach_request, "command") == "attach") {
     EmplaceSafeString(response, "message",
                       "Restarting an \"attach\" session is not supported.");
     g_vsc.SendJSON(llvm::json::Value(std::move(response)));
     return;
   }
 
   // The optional `arguments` field in RestartRequest can contain an updated
   // version of the launch arguments. If there's one, use it.
-  auto request_arguments = request.getObject("arguments");
-  if (request_arguments) {
-    llvm::json::Object arguments = *request_arguments;
+  auto restart_arguments = request.getObject("arguments");
+  if (restart_arguments) {
+    auto launch_request_arguments = restart_arguments->getObject("arguments");
+    if (launch_request_arguments) {
       (*g_vsc.last_launch_or_attach_request)["arguments"] =
-        llvm::json::Value(std::move(arguments));
+          llvm::json::Value(llvm::json::Object(*launch_request_arguments));
+    }
   }
 
   // Keep track of the old PID so when we get a "process exited" event from the
   // killed process we can detect it and not shut down the whole session.
   lldb::SBProcess process = g_vsc.target.GetProcess();
   g_vsc.restarting_process_id = process.GetProcessID();
 
   // Stop the current process if necessary. The logic here is similar to