Follow-up cleanups after clangd#728

 - The disposables passed in to install.activate() were not saved for
   cleanup
 - config.get() is not async

src/clangd-context.ts

@@ -57,7 +57,7 @@ class EnableEditsNearCursorFeature implements vscodelc.StaticFeature {
 }
 
 export class ClangdContext implements vscode.Disposable {
-  subscriptions: vscode.Disposable[] = [];
+  subscriptions: vscode.Disposable[];
   client: ClangdLanguageClient;
 
   static async create(globalStoragePath: string,
@@ -68,16 +68,15 @@ export class ClangdContext implements vscode.Disposable {
     if (!clangdPath)
       return null;
 
-    const clangdArguments = await config.get<string[]>('arguments');
-
-    return new ClangdContext(clangdPath, clangdArguments, outputChannel);
+    return new ClangdContext(subscriptions, clangdPath, outputChannel);
   }
 
-  private constructor(clangdPath: string, clangdArguments: string[],
+  private constructor(subscriptions: vscode.Disposable[], clangdPath: string,
                       outputChannel: vscode.OutputChannel) {
+    this.subscriptions = subscriptions;
     const clangd: vscodelc.Executable = {
       command: clangdPath,
-      args: clangdArguments,
+      args: config.get<string[]>('arguments'),
       options: {cwd: vscode.workspace.rootPath || process.cwd()}
     };
     const traceFile = config.get<string>('trace');