Fix: Instance vars should not be allowed on Class, Tuple, NamedTuple, Enum, Pointer , Proc, StaticArray and Union.

[I3oris]

Fix #12159.

The compiler now produce an error when user define an ivar on these types: Class, Tuple, NamedTuple, Enum, Pointer , Proc, StaticArray and Union.

class Class
  @x = 42
end
# => Error: can't declare instance variables in Class

(similar to previously triggered errors on forbidden types Int32 Number, Object...)

Note that TupleInstanceType or NamedTupleInstanceType still inherit from module InstanceVarContainer through GenericClassInstanceType, but couldn't touch that because GenericClassInstanceType is used by every generic class.

[beta-ziliani]

I think the direct enumeration is OK, as is a more abstract approach as suggested here.

Thanks @I3oris !

[I3oris]

Okay super!, thanks.