Add a check for arguments with optional basic types and a non-null default

[Gama11]

In almost all cases of code that looks like this, the author is not aware that ? implicitly turns basic types into Null<T> (non-basic types too, but that shouldn't really have any run-time implications for those).

function foo(?arg:Int = 0)  {} // the user probably didn't want a nullable type here

Nullable basic types on their own are fine, but them being nullable is pointless if they have a non-null default value as well. You can't get the argument to be null anyway (see here).

Note that there may be a use-case for nullable basic types with default-values if you want them to be skippable (skipping is only allowed for nullable arguments).