[Polly][Isl] Replacing isl method `to_str()` with `stringFromIslObj()`. NFC.

This is part of an effort to reduce the differences between the custom C++ bindings used right now by polly in `lib/External/isl/include/isl/isl-noxceptions.h` and the official isl C++ interface.

Changes made:
 - Removing method `to_str()` from all the classes in the isl C++ bindings.
 - Overload method `stringFromIslObj()` so it accepts isl C++ objects.
 - To keep backward compatibility `stringFromIslObj()` now accepts a value that is returned if the isl C object is `null` or doesn't have a string representation (by default it's an empty string). In some cases it's better to have the string "null" instead of an empty string.
 - isl-noexceptions.h has been generated by this https://github.com/patacca/isl/commit/d33ec3a3bbaf971b254a853e6d8200ff9f5a1385

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D104211