This patch makes tdesc_type an abstract base class and creates three
subclasses:
- tdesc_type_builtin, for builtin types
- tdesc_type_vector, for vector types
- tdesc_type_with_fields, for struct, union, flag and enum types
This allows getting rid of the union in tdesc_type and to not allow the
std::vector separately. I tried to go further and create separate
classes for struct, union, flag and enum, but it proved too difficult.
One problem is that from the point of the of the target description
code, the types tdesc_type_* are opaque (only forward-declared).
Therefore, it doesn't know about inheritance relationship between those
classes. This makes it impossible to make functions that accept a
pointer to a base class and pass a pointer to a derived class, for
example. I think this patch here is a good compromise, and if somebody
wants to improve things further, the door is open.
A make_gdb_type virtual pure method is added to tdesc_type, which
replaces the current tdesc_gdb_type function. Calling this method on a
tdesc_type returns the corresponding built gdb type.