Handle structs (unions?) with no fields

[hainest]

struct Foo {};
void foo(Foo f) {}

terminate called after throwing an instance of 'std::runtime_error'
what(): Can't allocate a {NO_CLASS, *}

Empty struct/class types are fairly common in C++. I think the solution is to not call the allocator when there are no fields in a structure.

[vsoch]

That makes sense - I can do a PR after we finish the current work we are doing.