C++: Fix resolving ui_* files in CppPreprocessor

The working copy contains the artificial ui_* files and therefore we
have to consider it while resolving include files.

Task-number: QTCREATORBUG-9683
Change-Id: Icb3387b4cd885b3652bae3f1da465d3e0f633332
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

tests/cppmodelmanager/testdata_guiproject1/main.cpp

@@ -0,0 +1,13 @@
+// Copyright license
+
+#include "mainwindow.h"
+#include <QApplication>
+
+int main(int argc, char *argv[])
+{
+    QApplication a(argc, argv);
+    MainWindow w;
+    w.show();
+
+    return a.exec();
+}