Previously if the type parameter was a pointer, it would assert that the
size of the type was resolved. It used to be that the size of pointers was
always resolved, however with lazy values, pointers gained the
possibility of not having their size resolved.

Now, type_allowed_in_extern triggers the resolution of whether a pointer
is zero bits, and returns a possible error if the resolution fails.

This fixes a compiler assertion when building the
[zootdeck project](https://github.com/donpdonp/zootdeck). I do not have
a test case reduction for the issue.