[Polly][Isl] Removing explicit operator bool() from isl C++ bindings. 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 explicit operator bool() from all the classes in the isl C++ bindings.
 - Replace each call to operator bool() to method `is_null()`.
 - isl-noexceptions.h has been generated by this https://github.com/patacca/isl/commit/27396daac5a5ee8228d25511a12f4a814c92ba8f

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D103976