ST6RI-881 Conditional computation of baseType of semantic metadata doesn't work

[seidewitz]

This PR corrects the evaluation of the feature value expression of the baseType of semantic metadata.

Consider the following KerML model:

class C;
struct S;

metaclass M :> Metaobjects::SemanticMetadata {
  :>> annotatedElement : KerML::Class
  :>> baseType = 
      if annotatedElement istype KerML::Structure ? 
          S meta SysML::Type else C meta SysML::Type;
}

#M struct T;

The structure T should have an implicit specialization to the base structure S, but, instead, it previously just received an implied specialization to Objects::Object. This is because the evaluation of the feature value expression for the baseType feature was computed using the annotatedElement T as the target, rather than the annotating metadata feature (where annotatedElement is a feature). The PR corrects this.