Clang: Watch directories instead of files

Because there a limited resources to watch files we watch now directories.
So we need much less resources.

Change-Id: Iac558832e9521a7a1a67c5ea99b42ad1b0b5129c
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/clangsupport/filesysteminterface.h

@@ -0,0 +1,43 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 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 "filepathid.h"
+
+#include <utils/smallstringview.h>
+
+namespace ClangBackEnd {
+
+class FileSystemInterface
+{
+public:
+    virtual FilePathIds directoryEntries(const QString &directoryPath) const = 0;
+    virtual long long lastModified(FilePathId filePathId) const = 0;
+
+protected:
+    ~FileSystemInterface() = default;
+};
+} // namespace ClangBackEnd