[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
This might have been there to ensure the order in which the objects are freed. LastSchedule must be free'd before IslCtx.
However, destruction order is defined to be in reverse of the field declaration order, so indeed what we want.