Fix missing DebugInformationFormat when symbols are set to full

[rorydriscoll]

When symbols are set to "full" then the DebugInformationFormat attribute is not emitted no matter what the other settings are. For me, this manifested itself as the editandcontinue function appearing not to work.

[samsinsane]

LGTM, might be worth changing this function to be a little less verbose one day. if cfg.symbols == p.OFF then and then if cfg.symbols then seems like it would be better than having to list all of the "On" or "Off" values.

[rorydriscoll]

It would be less error prone to explicitly list out the default case too and make the final else emit an error. That would have saved me a couple of hours. In another language this would be a switch statement with an error in the default case.