[Polly][Isl] Removing nullptr constructor from C++ bindings. NFC.

[Polly][Isl] Removing nullptr constructor from C++ bindings. NFC.

[Polly][Isl] Removing nullptr constructor from 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:
 - Removed `std::nullptr_t` constructor from all the classes in the isl C++ bindings.
 - `isl-noexceptions.h` has been generated by this https://github.com/patacca/isl/commit/a7e00bea38f251a4bcf5c2c6ce5fa7ee5f661528

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D103751