CppEditor: Proliferate FilePath use

This includes one functional change: It drops some cleaning
of the path used to create the CppDocument, which is now
assumed to be done on the caller side.

Change-Id: I5e2a182028e4d5b56282ad85f4a5c665f081754f
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

tests/auto/cplusplus/c99/tst_c99.cpp

@@ -73,7 +73,7 @@ class tst_c99: public QObject
 
     Document::Ptr document(const QString &fileName, QByteArray *errors = 0)
     {
-        Document::Ptr doc = Document::create(fileName);
+        Document::Ptr doc = Document::create(Utils::FilePath::fromString(fileName));
         QFile file(testdata(fileName));
         if (file.open(QFile::ReadOnly)) {
             LanguageFeatures features;