Processes the attribtue wildcard according to the spec.

Since the definition of the attribute wildcard is very adhoc, it cannot be naturally caputred by our AGM.

Therefore, when we parse a schema, we just parse <anyAttribute> directly. After all components are loaded, arcane computation is done to correctly compute the attribute wildcard.

Attribute wildcard will be ultimately converted into an expression, and that will be attached to the ComplexTypeExp#attWildcard.

This class also computes the attribute propagation that happens only when a complex type is derived by restriction. Consider the following fragment:

<complexType name="base">
  <attribute name="abc" ... />
</complexType>

<complexType name="derived">
  <complexContent>
    <restriction base="base"/>
  </complexContent>
</complexType>

According to the spec, the derived type will have the 'abc' attribute. By "propagation", we mean this behavior.