clear diagnostics on language server restart (#2817)

src/goLanguageServer.ts

@@ -244,6 +244,7 @@ export async function registerLanguageFeatures(ctx: vscode.ExtensionContext) {
 	ctx.subscriptions.push(languageServerDisposable);
 
 	ctx.subscriptions.push(vscode.commands.registerCommand('go.languageserver.restart', async () => {
+		c.diagnostics.clear();
 		await c.stop();
 		languageServerDisposable.dispose();
 		languageServerDisposable = c.start();