Clang: Introduce parallel indexing

Change-Id: I522cb18e6d24b7dbed5d5dfa3a732e5b3b5113bb
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/symbolindexertaskschedulerinterface.h

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+/****************************************************************************
+**
+** Copyright (C) 2018 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 <functional>
+#include <vector>
+
+namespace Sqlite {
+class TransactionInterface;
+}
+
+namespace ClangBackEnd {
+
+class SymbolsCollectorInterface;
+class SymbolStorageInterface;
+
+using uint = unsigned int;
+
+class SymbolIndexerTaskSchedulerInterface
+{
+public:
+    using Task = std::function<void(SymbolsCollectorInterface &symbolsCollector,
+                                    SymbolStorageInterface &symbolStorage,
+                                    Sqlite::TransactionInterface &transaction)>;
+
+    SymbolIndexerTaskSchedulerInterface() = default;
+    SymbolIndexerTaskSchedulerInterface(const SymbolIndexerTaskSchedulerInterface &) = delete;
+    SymbolIndexerTaskSchedulerInterface &operator=(const SymbolIndexerTaskSchedulerInterface &) = delete;
+
+    virtual void addTasks(std::vector<Task> &&tasks) = 0;
+    virtual uint freeSlots() = 0;
+
+protected:
+    ~SymbolIndexerTaskSchedulerInterface() = default;
+};
+
+} // namespace ClangBackEnd