C++: update project info once after finishing adding all project parts.

So that recalculating the information is not O(n^2) but linear.

Change-Id: I69903e0b5ad321d071804d782ad634a3f300e71a
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/qbsprojectmanager/qbsproject.cpp

@@ -741,6 +741,7 @@ void QbsProject::updateCppCodeModel(const qbs::ProjectData &prj)
 
     if (pinfo.projectParts().isEmpty())
         return;
+    pinfo.finish();
 
     QtSupport::UiCodeModelManager::update(this, uiFiles);