Closes #7451.

Given:

public interface IDataTrait {
    public associatedtype InputType;
}

public interface IBar {
    public void eval<Di : IDataTrait>(Di.InputType interface_input);
}

struct Impl : IBar
{
    public void eval<Dx : IDataTrait>(Dx.InputType impl_input);
}

The default declref to Impl.eval.Dx.InputType is LookupDeclRef(InputType, Dx, witness = DeclaredSubtypeWitness(declRef = DirectDeclRef(Impl.eval.Dx_is_IDataTrait)).

However when we try to match the implementation signature to interface signature, we will form a declref to IBar.eval.Di using substition Di = Dx, witness(Di:IDataTrait) = DeclaredSubtypeWitness(MemberDeclRef(GenericAppDeclRef(Impl.eval,DeclRefType(Dx)), Dx_is_IDataTrait), causing the pointer identity of the specialized declref to mismatch.

The fix is to exclude any default substitutions when forming a declref to the declared witness in the generic signature matching step.