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/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;
         }