Refactor the location classes to be attributes instead of separate IR classes.

Refactor the location classes to be attributes instead of separate IR classes.

This will allow for locations to be used in the same contexts as attributes. Given that attributes are nullable types, the 'Location' class now represents a non-nullable wrapper around a 'LocationAttr'. This preserves the desired semantics we have for non-optional locations.

PiperOrigin-RevId: 254505278