Fixed "@disable this" propagating through references (issue 8296)

As classes are reference types, disabeling the default constructor should not propagate through struct fields wich are actually references to objects.
https://d.puremagic.com/issues/show_bug.cgi?id=8296

Without this fix it is not allowed to instaniate bar1 using default construction. But this should be allowed as bar1 only contains a single field which is a reference and thus can be (default) initialized with null.