[clangd] Add "clangd.trace" VSCode setting to enable tracing.

[sammccall]

Summary

Setting the CLANGD_TRACE environment variable directly is awkward with VSCode's
"reload from the command palette" workflow.

[ilya-biryukov]

LGTM. We'll need to publish new version of VSCode extensions. +@hokein, who did that before.

[hokein]

Will make a new release for clangd vscode extension today.

clangd/clients/clangd-vscode/package.json

     "devDependencies": {
         "typescript": "^2.0.3",
         "vscode": "^1.0.3",
         "mocha": "^2.3.3",
         "@types/node": "^6.0.40",
         "@types/mocha": "^2.2.32"
     },
     "repository": {
         "type": "svn",
         "url": "http://llvm.org/svn/llvm-project/clang-tools-extra/trunk/clangd/clients/clangd-vscode/"
     },
     "contributes": {
         "configuration": {
             "type": "object",
             "title": "clangd configuration",
             "properties": {
                 "clangd.path": {
                     "type": "string",
                     "default": "clangd",
                     "description": "The path to clangd executable, e.g.: /usr/bin/clangd"
                 },
                 "clangd.arguments": {
                     "type": "array",
                     "default": [],
                     "items": {
                         "type": "string"
                     },
                     "description": "Arguments for clangd server"
                 },
                 "clangd.syncFileEvents": {
                     "type": "boolean",
                     "default": true,
                     "description": "Whether or not to send file events to clangd (File created, changed or deleted). This can be disabled for performance consideration."
+                },
+                "clangd.trace": {
+                    "type": "string",
+                    "description": "Names a file that clangd should log a performance trace to, in chrome trace-viewer JSON format."
                 }
             }
         }
     }
 }

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

function getConfig<T>(option: string, defaultValue?: any) : T {
     return config.get<T>(option, defaultValue);
 }
 
 /**
  *  this method is called when your extension is activate
  *  your extension is activated the very first time the command is executed
  */
 export function activate(context: vscode.ExtensionContext) {
-    const clangdPath = getConfig<string>('path');
-    const clangdArgs = getConfig<string[]>('arguments');
     const syncFileEvents = getConfig<boolean>('syncFileEvents', true);
 
-    const serverOptions: vscodelc.ServerOptions = { command: clangdPath, args: clangdArgs };
+    const clangd: vscodelc.Executable = {
+        command: getConfig<string>('path'),
+        args: getConfig<string[]>('arguments')
+    };
+    const traceFile = getConfig<string>('trace');
+    if (traceFile != null)
+        clangd.options = {env: {CLANGD_TRACE: traceFile}};
+    const serverOptions: vscodelc.ServerOptions = clangd;
 
     const filePattern: string = '**/*.{' +
       ['cpp', 'c', 'cc', 'cxx', 'c++', 'm', 'mm', 'h', 'hh', 'hpp', 'hxx', 'inc'].join() + '}';
     const clientOptions: vscodelc.LanguageClientOptions = {
         // Register the server for C/C++ files
         documentSelector: [{scheme: 'file', pattern: filePattern}],
         synchronize: !syncFileEvents ? undefined : {
             fileEvents: vscode.workspace.createFileSystemWatcher(filePattern)
         }
     };
 
     const clangdClient = new vscodelc.LanguageClient('Clang Language Server', serverOptions, clientOptions);
     console.log('Clang Language Server is now active!');
 
     const disposable = clangdClient.start();
 }