[sourcekitd] xpc_retain MainConnection so it remains valid for sending when cancelled from the other end

If a client crashes and we tried to post notifications to the XPC connection,
SourceKitService would go down too, as it never `xpc_retain`ed the connection.

Resolves rdar://problem/36077481

Author: Nathan Hawes

tools/SourceKit/tools/sourcekitd/bin/XPC/Service/XPCService.cpp

@@ -209,7 +209,7 @@ std::string sourcekitd::getRuntimeLibPath() {
 }
 
 static void sourcekitdServer_peer_event_handler(xpc_connection_t peer,
-                                            xpc_object_t event) {
+                                                xpc_object_t event) {
   xpc_type_t type = xpc_get_type(event);
   if (type == XPC_TYPE_ERROR) {
     if (event == XPC_ERROR_CONNECTION_INVALID) {
@@ -305,6 +305,10 @@ static void sourcekitdServer_event_handler(xpc_connection_t peer) {
     sourcekitdServer_peer_event_handler(peer, event);
   });
 
+  // Update the main connection
+  xpc_retain(peer);
+  if (MainConnection)
+    xpc_release(MainConnection);
   MainConnection = peer;
 
   // This will tell the connection to begin listening for events. If you