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/qmakeprojectmanager/qmakeproject.cpp

@@ -620,6 +620,7 @@ void QmakeProject::updateCppCodeModel()
         if (!cppPart->files.isEmpty())
             objcppPart->displayName += QLatin1String(" (ObjC++)");
     }
+    pinfo.finish();
 
     // Also update Ui Code Model Support:
     QtSupport::UiCodeModelManager::update(this, uiCodeModelData);