Add test for FixedStreamArrayIterator::operator->

The operator-> implementation comes from iterator_facade_base, so it should
just work given that the iterator has a tested operator*.  But r302257 showed
that required careful handling of for the const qualifier.  This patch ensures
the fix in r302257 doesn't regress.

Differential Revision: https://reviews.llvm.org/D33249

llvm-svn: 303215