210 – Strange results when covariantly implementing an interface method defined with an interface return type

This bug carries on where issue 65 left off. That issue covered the case where the interface-to-class covariance is in a class-to-class override. This one covers interface-to-class covariance in an interface-to-class override.
This was originally intended to be part of issue 65, but I figured that it might make more sense to break it off as a separate issue.
Testcases, based on issue 179 comment 1:
----------
import std.stdio;
interface Father {
 Father test();
 void showData();
}
class Child : Father {
 int data;
 this(int d) { data = d; }
 Child test() { 
 writefln("in CovFunc Test");
 return new Child(69);
 }
 void showData() {
 writefln("Child: %d", data);
 }
}
void icov2test() {
 Child aChild = new Child(42);
 aChild.showData();
 Father childsDad = aChild;
 childsDad.showData();
 Father dadTest = childsDad.test;
 writefln("FCALL dadTest.showData");
 dadTest.showData();
 writefln("FCALL dadTest.test");
 dadTest.test();
 writefln("FCALL (cast(Child) dadTest).showData");
 (cast(Child) dadTest).showData();
}
----------
import std.stdio;
interface IFoo {
}
interface ICov {
 IFoo covfunc();
}
class Child : ICov, IFoo {
 Child covfunc() { 
 writefln("in CovFunc Test");
 return new Child();
 }
}
void icov3() {
 ICov icov = new Child();
 IFoo ifoo = icov.covfunc();
 writefln(ifoo.classinfo.name); // Segfault or prints garbage
 writefln((cast(Object) ifoo)); // Segfault or prints garbage
}
----------
import std.stdio;
interface IFoo {
 IFoo covfunc();
}
class Child : IFoo {
 Child covfunc() { 
 writefln("in CovFunc Test");
 return new Child();
 }
}
void icov3() {
 IFoo icov = new Child();
 IFoo ifoo = icov.covfunc();
 writefln(ifoo.classinfo.name);
 writefln((cast(Object) ifoo));
}
----------
There are other cases to test, namely those involving interface-to-interface covariance || an interface-to-interface override. But getting the above tests working would be a start.

Fixed DMD 0.162