CppTools: Create ProjectInfos explicitly

...instead of going through CppModelManager::projectInfo().

Now CppModelManager::projectInfo() returns an invalid ProjectInfo in
case there is no valid data for the given Project.

Change-Id: I11908bf2ddf865b1d3d71ff176eaf4139292b21c
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>