Add validation for array size declaration with unresolved reference

[mhasel]

Is your feature request related to a problem? Please describe. Currently, initializing an array with an unresolved reference will be caught during the codegen phase:

VAR
    x : ARRAY[0..UNRESOLVED_REF] OF DINT;
END_VAR

We should add a validation for this and check to see if other unresolved references or non-constant references where we expect constants cause similar behaviour.