[clangd] Add activate command to the vscode extension.

[hokein]

Summary

This would help minizime the annoying part of not activating the extension
for .cu file.

[ilya-biryukov]

LGTM! Thanks!

clangd/clients/clangd-vscode/package.json

"keywords": [
         "LSP",
         "Clangd",
         "LLVM"
     ],
     "activationEvents": [
         "onLanguage:c",
         "onLanguage:cpp",
         "onLanguage:objective-c",
-        "onLanguage:objective-cpp"
+        "onLanguage:objective-cpp",
+        "onCommand:clangd-vscode.activate"
     ],
     "main": "./out/src/extension",
     "scripts": {
         "vscode:prepublish": "tsc -p ./",
         "compile": "tsc -watch -p ./",
         "postinstall": "node ./node_modules/vscode/bin/install",
         "test": "node ./node_modules/vscode/bin/test"
     },
"contributes": {
                     "description": "Names a file that clangd should log a performance trace to, in chrome trace-viewer JSON format."
                 }
             }
         },
         "commands": [
             {
                 "command": "clangd-vscode.switchheadersource",
                 "title": "Switch between Source/Header"
+            },
+            {
+                "command": "clangd-vscode.activate",
+                "title": "Manually activate clangd extension"
             }
         ],
         "keybindings": [
             {
                 "command": "clangd-vscode.switchheadersource",
                 "key": "Alt+o",
                 "mac": "Alt+cmd+o",
                 "when": "editorTextFocus"
             }
         ]
     }
 }

clangd/clients/clangd-vscode/src/extension.ts

clangdClient.onDidChangeState(
                 clangdClient.onNotification(
                     'textDocument/clangd.fileStatus',
                     (fileStatus) => { status.onFileUpdated(fileStatus); });
             } else if (newState == vscodelc.State.Stopped) {
                 // Clear all cached statuses when clangd crashes.
                 status.clear();
             }
         })
+    // An empty place holder for the activate command, otherwise we'll get an
+    // "command is not registered" error.
+    context.subscriptions.push(vscode.commands.registerCommand(
+            'clangd-vscode.activate', async () => {}));
 }