CppEditor: Make sure fallback project part is up to date

... with regards to the session include paths.
Amends 0636238429.

Fixes: QTCREATORBUG-26323
Change-Id: I8e2cd5f5e87d9dc3d2df3f943e13599bc7139768
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangmodelmanagersupport.cpp

@@ -483,10 +483,6 @@ void ClangModelManagerSupport::onEditorOpened(Core::IEditor *editor)
 
         // TODO: Ensure that not fully loaded documents are updated?
 
-        // TODO: If the file does not belong to any project and it is a header file,
-        //       it might make sense to check whether the file is included by any file
-        //       that does belong to a project, and if so, use the respective client
-        //       instead. Is this feasible?
         ProjectExplorer::Project * const project
                 = ProjectExplorer::SessionManager::projectForFile(document->filePath());
         if (ClangdClient * const client = clientForProject(project))

src/plugins/cppeditor/cppmodelmanager.cpp

@@ -694,6 +694,10 @@ CppModelManager::CppModelManager()
 
     connect(KitManager::instance(), &KitManager::kitsChanged, this,
             &CppModelManager::setupFallbackProjectPart);
+    connect(this, &CppModelManager::projectPartsRemoved, this,
+            &CppModelManager::setupFallbackProjectPart);
+    connect(this, &CppModelManager::projectPartsUpdated, this,
+            &CppModelManager::setupFallbackProjectPart);
     setupFallbackProjectPart();
 
     qRegisterMetaType<CPlusPlus::Document::Ptr>("CPlusPlus::Document::Ptr");