[python] don't try to free "independent" types

[Tom Tromey]

>>>>> "Thiago" == Thiago Jung Bauermann <bauerman@br.ibm.com> writes:

Thiago> Some type hierarchies are also created in places that call
Thiago> init_composite_type.

Thanks.

Thiago> Also, IIUC you'd need some checks to avoid freeing builtin_types, and
Thiago> types created by each language, and types created by each architecture
Thiago> which they use for their fancy registers, and other eventual types which
Thiago> are not initially associated with an objfile.

We can have the creator acquire a reference and never release it.

Thiago> Is it possible that one struct type can be present in two hierarchies?
Thiago> From some initial thinking, I believe this can only happen for the
Thiago> special cases I mentioned above.

I was wondering about range types for arrays.  That was the only case
I could think of.

Tom