Implement an iterator for isl maps, basic_maps, sets, basic_sets

Summary:
Provide an iterator to simplify iteration over some isl collections.
Since these types do not natively support iteration, they have to be converted
to an list first by the caller, but can then be used in a ranged for loop:
```
isl::set S;
for (auto SubSet : S.get_basic_set_list ()) {
  // ...
}
```

Reviewers: bollu, Meinersbur, grosser, dexonsmith

Reviewed By: bollu

Subscribers: hfinkel, mgorny, Meinersbur, mehdi_amini, bollu, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D48136

llvm-svn: 335951