[cpp] Recursive structs in a template context

[intrigus-lgtm]

Unfortunately this is still not a complete fix :(

template <typename> struct a;
struct b;
template <typename> struct a { b *c; };
struct b : a<int> {};

(Basically the original testcase but this time with templates)

Fails again with

Assertion `type.id() == typet::t_struct || type.id() == typet::t_union || type.id() == typet::t_class' failed.

Originally posted by @intrigus-lgtm in #1781 (comment)