Support for self recursive constants in Uniontypes

[johti17]

Ticket #5944 added support for constants in Uniontypes.
While this works for constants such as

uniontype UT
 constant Integer ICONST = 1;
 record R
 end R;
end UT;

It does not work with self recursive definitions, which currently work for function inputs and outputs.

For instance:

uniontype UT
 constant UT RCONST = R1();
 record R1
 end R1;

 record R2
 end R2;
end UT;

The goal of this ticket is to add this functionality

[johti17]

Fixed in ​https://github.com/OpenModelica/OpenModelica/pull/890