Avoid storing references to rvalue FileName strings

Following 73616849, it is dangerous to store a reference to the rvalue of
toString(), since it might become dangling.

Example:

FileName foo() { return FileName::fromString(QLatin1String("/some/file")); }

void func()
{
    const QString &fileName = foo().toString();
    // fileName is now a dangling reference
}

Change-Id: I5dfad5dc8dd568a0a3c8f9f71ad93292dc26cbbe
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/coreplugin/editormanager/editormanager.cpp

@@ -1662,7 +1662,7 @@ bool EditorManagerPrivate::saveDocument(IDocument *document)
 
     document->checkPermissions();
 
-    const QString &fileName = document->filePath().toString();
+    const QString fileName = document->filePath().toString();
 
     if (fileName.isEmpty())
         return saveDocumentAs(document);

src/plugins/projectexplorer/session.cpp

@@ -490,7 +490,7 @@ QStringList SessionManagerPrivate::dependenciesOrder() const
 
     // copy the map to a temporary list
     foreach (Project *pro, m_projects) {
-        const QString &proName = pro->projectFilePath().toString();
+        const QString proName = pro->projectFilePath().toString();
         unordered << QPair<QString, QStringList>(proName, m_depMap.value(proName));
     }
 

src/plugins/texteditor/texteditor.cpp

@@ -7201,7 +7201,7 @@ void TextEditorWidget::configureGenericHighlighter()
                 setCodeFoldingSupported(true);
             }
         } else {
-            const QString &fileName = textDocument()->filePath().toString();
+            const QString fileName = textDocument()->filePath().toString();
             if (TextEditorSettings::highlighterSettings().isIgnoredFilePattern(fileName))
                 d->m_isMissingSyntaxDefinition = false;
         }