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>