Clang: Significantly improve ClangFormat plugin usability

What's new:
1. New LibFormat option is used to prevent lines shrink,
   which allows to drop most of tricks used before for that purpose.
2. Cached UTF-8 source code is used to improve performance
3. Improved error handling.
4. Slightly improved UI.

Change-Id: I4605200fa103167369a40650b2e1ad2c61e8133b
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/clangformat/clangformatutils.h

@@ -0,0 +1,51 @@
+/****************************************************************************
+**
+** 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 <utils/fileutils.h>
+#include <clang/Format/Format.h>
+
+#include <QFile>
+
+#include <fstream>
+
+namespace ClangFormat {
+
+inline void createStyleFileIfNeeded(Utils::FileName styleConfigPath)
+{
+    const QString configFile = styleConfigPath.appendPath(".clang-format").toString();
+    if (QFile::exists(configFile))
+        return;
+
+    clang::format::FormatStyle newStyle = clang::format::getLLVMStyle();
+    std::fstream newStyleFile(configFile.toStdString(), std::fstream::out);
+    if (newStyleFile.is_open()) {
+        newStyleFile << clang::format::configurationAsText(newStyle);
+        newStyleFile.close();
+    }
+}
+
+}