Clang: Process distinct documents concurrently

Speed ups the typical use cases that can profit from this:
 * Change a header file and switch then to source file
 * Open documents one after the other (Follow Symbol)
 * Change documents visible in splits (e.g. by refactoring action)
 * Restore a session with multiple splits

Fixes the test ClangIpcServer.GetCodeCompletionDependingOnArgumets.

Change-Id: Ia575bd59780df14146dfc091a4d48794e4a0543d
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangbackend/ipcsource/clangasyncjob.h

@@ -0,0 +1,72 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "clangiasyncjob.h"
+
+#include <utils/runextensions.h>
+
+#include <QFutureWatcher>
+#include <QObject>
+
+namespace ClangBackEnd {
+
+template<class Result>
+class AsyncJob : public IAsyncJob
+{
+public:
+    AsyncJob() {}
+    ~AsyncJob() {}
+
+    using Runner = std::function<Result()>;
+    Runner runner() const { return m_runner; }
+    void setRunner(const Runner &runner) { m_runner = runner; }
+
+    Result asyncResult() const { return m_futureWatcher.future().result(); }
+
+    QFuture<void> runAsync() override
+    {
+        const auto onFinished = [this]() {
+            finalizeAsyncRun();
+            setIsFinished(true);
+            finishedHandler()(this);
+        };
+        QObject::connect(&m_futureWatcher,
+                         &QFutureWatcher<Result>::finished,
+                         onFinished);
+
+        const QFuture<Result> future = Utils::runAsync(m_runner);
+        m_futureWatcher.setFuture(future);
+
+        return future;
+    }
+
+private:
+    Runner m_runner;
+    QFutureWatcher<Result> m_futureWatcher;
+};
+
+} // namespace ClangBackEnd