Disable config file watcher if a hot-reload capability is set (#128)

Corresponds to https://reviews.llvm.org/D94222

This is the minimal change. I have a feeling the dispose/re-watch logic could be made cleaner.
But this client-side feature can just be removed in future, so I'm not sure it's worth the risk of trying to polish it.

I've tested this locally with clangd with/without the capability.

package.json

@@ -132,7 +132,7 @@
                 "clangd.onConfigChanged": {
                     "type": "string",
                     "default": "prompt",
-                    "description": "What to do when clangd configuration files are changed.",
+                    "description": "What to do when clangd configuration files are changed. Ignored for clangd 12+, which can reload such files itself.",
                     "enum": [
                         "prompt",
                         "restart",

src/config-file-watcher.ts

@@ -1,18 +1,43 @@
 import * as vscode from 'vscode';
+import * as vscodelc from 'vscode-languageclient/node';
 
 import {ClangdContext} from './clangd-context';
 import * as config from './config';
 
 export function activate(context: ClangdContext) {
   if (config.get<string>('onConfigChanged') != 'ignore') {
-    const watcher = new ConfigFileWatcher(context);
+    context.client.registerFeature(new ConfigFileWatcherFeature(context));
   }
 }
 
-class ConfigFileWatcher {
+// Clangd extension capabilities.
+interface ClangdClientCapabilities {
+  compilationDatabase?: {automaticReload?: boolean;},
+}
+
+class ConfigFileWatcherFeature implements vscodelc.StaticFeature {
+  constructor(private context: ClangdContext) {}
+  fillClientCapabilities(capabilities: vscodelc.ClientCapabilities) {}
+
+  initialize(capabilities: vscodelc.ServerCapabilities,
+             _documentSelector: vscodelc.DocumentSelector|undefined) {
+    if ((capabilities as ClangdClientCapabilities)
+            .compilationDatabase?.automaticReload)
+      return;
+    this.context.subscriptions.push(new ConfigFileWatcher(this.context));
+  }
+  dispose() {}
+}
+
+class ConfigFileWatcher implements vscode.Disposable {
   private databaseWatcher: vscode.FileSystemWatcher = undefined;
   private debounceTimer: NodeJS.Timer = undefined;
 
+  dispose() {
+    if (this.databaseWatcher)
+      this.databaseWatcher.dispose();
+  }
+
   constructor(private context: ClangdContext) {
     this.createFileSystemWatcher();
     context.subscriptions.push(vscode.workspace.onDidChangeWorkspaceFolders(