Fix incorrect inheritance declaration in IndexIterator

The BitVector::IndexIterator was declared as inheriting from
std::iterator without any access modifiers. This prevented the
inherited declarations from being visible, meaning it could not be
used by much of our other iterator code. Declare this inheritance
public and add a test that the iterator can be used with
TransformIterator.

Test: ./test.py --host
Change-Id: I7e6483e03d01fb447dd73f80a72173190cc5fe60
2 files changed