Add the `-Wall` option that enables all warnings

[Linyxus]

fixes #18559

This PR adds the -Wall option, which enables all warnings when it is set. Note that when -Wunused is not set (empty), setting -Wall is equivalent to passing -Wunused:all. But when -Wunused is set to some specific value (e.g. -Wunused:imports,locals), passing -Wall does not change it.