there were fixmes for this already

i am about to remove is_ptr (since callers need to properly distinguish
between pointers in different address spaces), so might as well do this
at the same time

...and remove it from `PointeeInfo`, which isn't meant for this.

There are still various places (marked with FIXMEs) that assume all pointers
have the same size and alignment. Fixing this requires parsing non-default
address spaces in the data layout string, which will be done in a followup.